diff --git a/google/cloud/documentai/__init__.py b/google/cloud/documentai/__init__.py
index a14d0857..137d58e6 100644
--- a/google/cloud/documentai/__init__.py
+++ b/google/cloud/documentai/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/__init__.py b/google/cloud/documentai_v1beta2/__init__.py
index f855da07..63c78042 100644
--- a/google/cloud/documentai_v1beta2/__init__.py
+++ b/google/cloud/documentai_v1beta2/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/services/__init__.py b/google/cloud/documentai_v1beta2/services/__init__.py
index 2c56c537..42ffdf2b 100644
--- a/google/cloud/documentai_v1beta2/services/__init__.py
+++ b/google/cloud/documentai_v1beta2/services/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/services/document_understanding_service/__init__.py b/google/cloud/documentai_v1beta2/services/document_understanding_service/__init__.py
index 8e39d0b8..52e41717 100644
--- a/google/cloud/documentai_v1beta2/services/document_understanding_service/__init__.py
+++ b/google/cloud/documentai_v1beta2/services/document_understanding_service/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/services/document_understanding_service/client.py b/google/cloud/documentai_v1beta2/services/document_understanding_service/client.py
index 1035b8c5..e251fd78 100644
--- a/google/cloud/documentai_v1beta2/services/document_understanding_service/client.py
+++ b/google/cloud/documentai_v1beta2/services/document_understanding_service/client.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -16,7 +16,8 @@
 #
 
 from collections import OrderedDict
-from typing import Dict, Sequence, Tuple, Type, Union
+import re
+from typing import Callable, Dict, Sequence, Tuple, Type, Union
 import pkg_resources
 
 import google.api_core.client_options as ClientOptions  # type: ignore
@@ -77,8 +78,38 @@ class DocumentUnderstandingServiceClient(
     as natural language, computer vision, and translation.
     """
 
-    DEFAULT_OPTIONS = ClientOptions.ClientOptions(
-        api_endpoint="us-documentai.googleapis.com"
+    @staticmethod
+    def _get_default_mtls_endpoint(api_endpoint):
+        """Convert api endpoint to mTLS endpoint.
+        Convert "*.sandbox.googleapis.com" and "*.googleapis.com" to
+        "*.mtls.sandbox.googleapis.com" and "*.mtls.googleapis.com" respectively.
+        Args:
+            api_endpoint (Optional[str]): the api endpoint to convert.
+        Returns:
+            str: converted mTLS api endpoint.
+        """
+        if not api_endpoint:
+            return api_endpoint
+
+        mtls_endpoint_re = re.compile(
+            r"(?P<name>[^.]+)(?P<mtls>\.mtls)?(?P<sandbox>\.sandbox)?(?P<googledomain>\.googleapis\.com)?"
+        )
+
+        m = mtls_endpoint_re.match(api_endpoint)
+        name, mtls, sandbox, googledomain = m.groups()
+        if mtls or not googledomain:
+            return api_endpoint
+
+        if sandbox:
+            return api_endpoint.replace(
+                "sandbox.googleapis.com", "mtls.sandbox.googleapis.com"
+            )
+
+        return api_endpoint.replace(".googleapis.com", ".mtls.googleapis.com")
+
+    DEFAULT_ENDPOINT = "us-documentai.googleapis.com"
+    DEFAULT_MTLS_ENDPOINT = _get_default_mtls_endpoint.__func__(  # type: ignore
+        DEFAULT_ENDPOINT
     )
 
     @classmethod
@@ -106,7 +137,7 @@ def __init__(
         *,
         credentials: credentials.Credentials = None,
         transport: Union[str, DocumentUnderstandingServiceTransport] = None,
-        client_options: ClientOptions = DEFAULT_OPTIONS,
+        client_options: ClientOptions = None,
     ) -> None:
         """Instantiate the document understanding service client.
 
@@ -120,6 +151,17 @@ def __init__(
                 transport to use. If set to None, a transport is chosen
                 automatically.
             client_options (ClientOptions): Custom options for the client.
+                (1) The ``api_endpoint`` property can be used to override the
+                default endpoint provided by the client.
+                (2) If ``transport`` argument is None, ``client_options`` can be
+                used to create a mutual TLS transport. If ``client_cert_source``
+                is provided, mutual TLS transport will be created with the given
+                ``api_endpoint`` or the default mTLS endpoint, and the client
+                SSL credentials obtained from ``client_cert_source``.
+
+        Raises:
+            google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
+                creation failed for any reason.
         """
         if isinstance(client_options, dict):
             client_options = ClientOptions.from_dict(client_options)
@@ -128,17 +170,46 @@ def __init__(
         # Ordinarily, we provide the transport, but allowing a custom transport
         # instance provides an extensibility point for unusual situations.
         if isinstance(transport, DocumentUnderstandingServiceTransport):
+            # transport is a DocumentUnderstandingServiceTransport instance.
             if credentials:
                 raise ValueError(
                     "When providing a transport instance, "
                     "provide its credentials directly."
                 )
             self._transport = transport
-        else:
+        elif client_options is None or (
+            client_options.api_endpoint is None
+            and client_options.client_cert_source is None
+        ):
+            # Don't trigger mTLS if we get an empty ClientOptions.
             Transport = type(self).get_transport_class(transport)
             self._transport = Transport(
+                credentials=credentials, host=self.DEFAULT_ENDPOINT
+            )
+        else:
+            # We have a non-empty ClientOptions. If client_cert_source is
+            # provided, trigger mTLS with user provided endpoint or the default
+            # mTLS endpoint.
+            if client_options.client_cert_source:
+                api_mtls_endpoint = (
+                    client_options.api_endpoint
+                    if client_options.api_endpoint
+                    else self.DEFAULT_MTLS_ENDPOINT
+                )
+            else:
+                api_mtls_endpoint = None
+
+            api_endpoint = (
+                client_options.api_endpoint
+                if client_options.api_endpoint
+                else self.DEFAULT_ENDPOINT
+            )
+
+            self._transport = DocumentUnderstandingServiceGrpcTransport(
                 credentials=credentials,
-                host=client_options.api_endpoint or "us-documentai.googleapis.com",
+                host=api_endpoint,
+                api_mtls_endpoint=api_mtls_endpoint,
+                client_cert_source=client_options.client_cert_source,
             )
 
     def batch_process_documents(
@@ -195,6 +266,7 @@ def batch_process_documents(
 
         # If we have keyword arguments corresponding to fields on the
         # request, apply these.
+
         if requests is not None:
             request.requests = requests
 
diff --git a/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/__init__.py b/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/__init__.py
index f3a91e0d..7030f9c6 100644
--- a/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/__init__.py
+++ b/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/base.py b/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/base.py
index 956f3a2c..6809b5da 100644
--- a/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/base.py
+++ b/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/base.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/grpc.py b/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/grpc.py
index 80519aa8..b021b9e5 100644
--- a/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/grpc.py
+++ b/google/cloud/documentai_v1beta2/services/document_understanding_service/transports/grpc.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -15,11 +15,13 @@
 # limitations under the License.
 #
 
-from typing import Callable, Dict
+from typing import Callable, Dict, Tuple
 
 from google.api_core import grpc_helpers  # type: ignore
 from google.api_core import operations_v1  # type: ignore
 from google.auth import credentials  # type: ignore
+from google.auth.transport.grpc import SslCredentials  # type: ignore
+
 
 import grpc  # type: ignore
 
@@ -50,7 +52,9 @@ def __init__(
         *,
         host: str = "us-documentai.googleapis.com",
         credentials: credentials.Credentials = None,
-        channel: grpc.Channel = None
+        channel: grpc.Channel = None,
+        api_mtls_endpoint: str = None,
+        client_cert_source: Callable[[], Tuple[bytes, bytes]] = None
     ) -> None:
         """Instantiate the transport.
 
@@ -64,20 +68,55 @@ def __init__(
                 This argument is ignored if ``channel`` is provided.
             channel (Optional[grpc.Channel]): A ``Channel`` instance through
                 which to make calls.
+            api_mtls_endpoint (Optional[str]): The mutual TLS endpoint. If
+                provided, it overrides the ``host`` argument and tries to create
+                a mutual TLS channel with client SSL credentials from
+                ``client_cert_source`` or applicatin default SSL credentials.
+            client_cert_source (Optional[Callable[[], Tuple[bytes, bytes]]]): A
+                callback to provide client SSL certificate bytes and private key
+                bytes, both in PEM format. It is ignored if ``api_mtls_endpoint``
+                is None.
+
+        Raises:
+          google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
+              creation failed for any reason.
         """
-        # Sanity check: Ensure that channel and credentials are not both
-        # provided.
         if channel:
+            # Sanity check: Ensure that channel and credentials are not both
+            # provided.
             credentials = False
 
+            # If a channel was explicitly provided, set it.
+            self._grpc_channel = channel
+        elif api_mtls_endpoint:
+            host = (
+                api_mtls_endpoint
+                if ":" in api_mtls_endpoint
+                else api_mtls_endpoint + ":443"
+            )
+
+            # Create SSL credentials with client_cert_source or application
+            # default SSL credentials.
+            if client_cert_source:
+                cert, key = client_cert_source()
+                ssl_credentials = grpc.ssl_channel_credentials(
+                    certificate_chain=cert, private_key=key
+                )
+            else:
+                ssl_credentials = SslCredentials().ssl_credentials
+
+            # create a new channel. The provided one is ignored.
+            self._grpc_channel = grpc_helpers.create_channel(
+                host,
+                credentials=credentials,
+                ssl_credentials=ssl_credentials,
+                scopes=self.AUTH_SCOPES,
+            )
+
         # Run the base constructor.
         super().__init__(host=host, credentials=credentials)
         self._stubs = {}  # type: Dict[str, Callable]
 
-        # If a channel was explicitly provided, set it.
-        if channel:
-            self._grpc_channel = channel
-
     @classmethod
     def create_channel(
         cls,
diff --git a/google/cloud/documentai_v1beta2/types/__init__.py b/google/cloud/documentai_v1beta2/types/__init__.py
index c22af483..eeb87607 100644
--- a/google/cloud/documentai_v1beta2/types/__init__.py
+++ b/google/cloud/documentai_v1beta2/types/__init__.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/types/document.py b/google/cloud/documentai_v1beta2/types/document.py
index d9ca295f..086ee8bf 100644
--- a/google/cloud/documentai_v1beta2/types/document.py
+++ b/google/cloud/documentai_v1beta2/types/document.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/types/document_understanding.py b/google/cloud/documentai_v1beta2/types/document_understanding.py
index 3b36d129..321344ae 100644
--- a/google/cloud/documentai_v1beta2/types/document_understanding.py
+++ b/google/cloud/documentai_v1beta2/types/document_understanding.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/google/cloud/documentai_v1beta2/types/geometry.py b/google/cloud/documentai_v1beta2/types/geometry.py
index 69166b38..a5173fa1 100644
--- a/google/cloud/documentai_v1beta2/types/geometry.py
+++ b/google/cloud/documentai_v1beta2/types/geometry.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
diff --git a/setup.py b/setup.py
index 1645c0ed..38cbf73f 100644
--- a/setup.py
+++ b/setup.py
@@ -41,9 +41,7 @@
     platforms="Posix; MacOS X; Windows",
     include_package_data=True,
     install_requires=(
-        "google-api-core >= 1.8.0, < 2.0.0dev",
-        "googleapis-common-protos >= 1.5.8",
-        "grpcio >= 1.10.0",
+        "google-api-core[grpc] >= 1.17.0, < 2.0.0dev",
         "proto-plus >= 0.4.0",
     ),
     python_requires=">=3.6",
diff --git a/synth.metadata b/synth.metadata
index 0679d4de..b2198590 100644
--- a/synth.metadata
+++ b/synth.metadata
@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-documentai.git",
-        "sha": "00f93cf5808b81e82b60493a674ed8b3b2184bf1"
+        "sha": "75374500a3c1e22d0d9ab74323b142f972edd78c"
       }
     },
     {
diff --git a/tests/unit/documentai_v1beta2/test_document_understanding_service.py b/tests/unit/documentai_v1beta2/test_document_understanding_service.py
index 8a442050..2d9ff8f0 100644
--- a/tests/unit/documentai_v1beta2/test_document_understanding_service.py
+++ b/tests/unit/documentai_v1beta2/test_document_understanding_service.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright (C) 2019  Google LLC
+# Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -24,6 +24,7 @@
 from google import auth
 from google.api_core import client_options
 from google.api_core import future
+from google.api_core import grpc_helpers
 from google.api_core import operations_v1
 from google.auth import credentials
 from google.cloud.documentai_v1beta2.services.document_understanding_service import (
@@ -34,11 +35,48 @@
 )
 from google.cloud.documentai_v1beta2.types import document
 from google.cloud.documentai_v1beta2.types import document_understanding
+from google.cloud.documentai_v1beta2.types import geometry
 from google.longrunning import operations_pb2
 from google.oauth2 import service_account
 from google.rpc import status_pb2 as status  # type: ignore
 
 
+def client_cert_source_callback():
+    return b"cert bytes", b"key bytes"
+
+
+def test__get_default_mtls_endpoint():
+    api_endpoint = "example.googleapis.com"
+    api_mtls_endpoint = "example.mtls.googleapis.com"
+    sandbox_endpoint = "example.sandbox.googleapis.com"
+    sandbox_mtls_endpoint = "example.mtls.sandbox.googleapis.com"
+    non_googleapi = "api.example.com"
+
+    assert DocumentUnderstandingServiceClient._get_default_mtls_endpoint(None) is None
+    assert (
+        DocumentUnderstandingServiceClient._get_default_mtls_endpoint(api_endpoint)
+        == api_mtls_endpoint
+    )
+    assert (
+        DocumentUnderstandingServiceClient._get_default_mtls_endpoint(api_mtls_endpoint)
+        == api_mtls_endpoint
+    )
+    assert (
+        DocumentUnderstandingServiceClient._get_default_mtls_endpoint(sandbox_endpoint)
+        == sandbox_mtls_endpoint
+    )
+    assert (
+        DocumentUnderstandingServiceClient._get_default_mtls_endpoint(
+            sandbox_mtls_endpoint
+        )
+        == sandbox_mtls_endpoint
+    )
+    assert (
+        DocumentUnderstandingServiceClient._get_default_mtls_endpoint(non_googleapi)
+        == non_googleapi
+    )
+
+
 def test_document_understanding_service_client_from_service_account_file():
     creds = credentials.AnonymousCredentials()
     with mock.patch.object(
@@ -59,31 +97,89 @@ def test_document_understanding_service_client_from_service_account_file():
 
 
 def test_document_understanding_service_client_client_options():
-    # Check the default options have their expected values.
-    assert (
-        DocumentUnderstandingServiceClient.DEFAULT_OPTIONS.api_endpoint
-        == "us-documentai.googleapis.com"
-    )
+    # Check that if channel is provided we won't create a new one.
+    with mock.patch(
+        "google.cloud.documentai_v1beta2.services.document_understanding_service.DocumentUnderstandingServiceClient.get_transport_class"
+    ) as gtc:
+        transport = transports.DocumentUnderstandingServiceGrpcTransport(
+            credentials=credentials.AnonymousCredentials()
+        )
+        client = DocumentUnderstandingServiceClient(transport=transport)
+        gtc.assert_not_called()
 
-    # Check that options can be customized.
-    options = client_options.ClientOptions(api_endpoint="squid.clam.whelk")
+    # Check mTLS is not triggered with empty client options.
+    options = client_options.ClientOptions()
     with mock.patch(
         "google.cloud.documentai_v1beta2.services.document_understanding_service.DocumentUnderstandingServiceClient.get_transport_class"
     ) as gtc:
         transport = gtc.return_value = mock.MagicMock()
         client = DocumentUnderstandingServiceClient(client_options=options)
-        transport.assert_called_once_with(credentials=None, host="squid.clam.whelk")
+        transport.assert_called_once_with(
+            credentials=None, host=client.DEFAULT_ENDPOINT
+        )
+
+    # Check mTLS is not triggered if api_endpoint is provided but
+    # client_cert_source is None.
+    options = client_options.ClientOptions(api_endpoint="squid.clam.whelk")
+    with mock.patch(
+        "google.cloud.documentai_v1beta2.services.document_understanding_service.transports.DocumentUnderstandingServiceGrpcTransport.__init__"
+    ) as grpc_transport:
+        grpc_transport.return_value = None
+        client = DocumentUnderstandingServiceClient(client_options=options)
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint=None,
+            client_cert_source=None,
+            credentials=None,
+            host="squid.clam.whelk",
+        )
+
+    # Check mTLS is triggered if client_cert_source is provided.
+    options = client_options.ClientOptions(
+        client_cert_source=client_cert_source_callback
+    )
+    with mock.patch(
+        "google.cloud.documentai_v1beta2.services.document_understanding_service.transports.DocumentUnderstandingServiceGrpcTransport.__init__"
+    ) as grpc_transport:
+        grpc_transport.return_value = None
+        client = DocumentUnderstandingServiceClient(client_options=options)
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint=client.DEFAULT_MTLS_ENDPOINT,
+            client_cert_source=client_cert_source_callback,
+            credentials=None,
+            host=client.DEFAULT_ENDPOINT,
+        )
+
+    # Check mTLS is triggered if api_endpoint and client_cert_source are provided.
+    options = client_options.ClientOptions(
+        api_endpoint="squid.clam.whelk", client_cert_source=client_cert_source_callback
+    )
+    with mock.patch(
+        "google.cloud.documentai_v1beta2.services.document_understanding_service.transports.DocumentUnderstandingServiceGrpcTransport.__init__"
+    ) as grpc_transport:
+        grpc_transport.return_value = None
+        client = DocumentUnderstandingServiceClient(client_options=options)
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint="squid.clam.whelk",
+            client_cert_source=client_cert_source_callback,
+            credentials=None,
+            host="squid.clam.whelk",
+        )
 
 
 def test_document_understanding_service_client_client_options_from_dict():
     with mock.patch(
-        "google.cloud.documentai_v1beta2.services.document_understanding_service.DocumentUnderstandingServiceClient.get_transport_class"
-    ) as gtc:
-        transport = gtc.return_value = mock.MagicMock()
+        "google.cloud.documentai_v1beta2.services.document_understanding_service.transports.DocumentUnderstandingServiceGrpcTransport.__init__"
+    ) as grpc_transport:
+        grpc_transport.return_value = None
         client = DocumentUnderstandingServiceClient(
             client_options={"api_endpoint": "squid.clam.whelk"}
         )
-        transport.assert_called_once_with(credentials=None, host="squid.clam.whelk")
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint=None,
+            client_cert_source=None,
+            credentials=None,
+            host="squid.clam.whelk",
+        )
 
 
 def test_batch_process_documents(transport: str = "grpc"):
@@ -279,8 +375,87 @@ def test_document_understanding_service_host_with_port():
 
 def test_document_understanding_service_grpc_transport_channel():
     channel = grpc.insecure_channel("http://localhost/")
-    transport = transports.DocumentUnderstandingServiceGrpcTransport(channel=channel)
-    assert transport.grpc_channel is channel
+
+    # Check that if channel is provided, mtls endpoint and client_cert_source
+    # won't be used.
+    callback = mock.MagicMock()
+    transport = transports.DocumentUnderstandingServiceGrpcTransport(
+        host="squid.clam.whelk",
+        channel=channel,
+        api_mtls_endpoint="mtls.squid.clam.whelk",
+        client_cert_source=callback,
+    )
+    assert transport.grpc_channel == channel
+    assert transport._host == "squid.clam.whelk:443"
+    assert not callback.called
+
+
+@mock.patch("grpc.ssl_channel_credentials", autospec=True)
+@mock.patch("google.api_core.grpc_helpers.create_channel", autospec=True)
+def test_document_understanding_service_grpc_transport_channel_mtls_with_client_cert_source(
+    grpc_create_channel, grpc_ssl_channel_cred
+):
+    # Check that if channel is None, but api_mtls_endpoint and client_cert_source
+    # are provided, then a mTLS channel will be created.
+    mock_cred = mock.Mock()
+
+    mock_ssl_cred = mock.Mock()
+    grpc_ssl_channel_cred.return_value = mock_ssl_cred
+
+    mock_grpc_channel = mock.Mock()
+    grpc_create_channel.return_value = mock_grpc_channel
+
+    transport = transports.DocumentUnderstandingServiceGrpcTransport(
+        host="squid.clam.whelk",
+        credentials=mock_cred,
+        api_mtls_endpoint="mtls.squid.clam.whelk",
+        client_cert_source=client_cert_source_callback,
+    )
+    grpc_ssl_channel_cred.assert_called_once_with(
+        certificate_chain=b"cert bytes", private_key=b"key bytes"
+    )
+    grpc_create_channel.assert_called_once_with(
+        "mtls.squid.clam.whelk:443",
+        credentials=mock_cred,
+        ssl_credentials=mock_ssl_cred,
+        scopes=("https://www.googleapis.com/auth/cloud-platform",),
+    )
+    assert transport.grpc_channel == mock_grpc_channel
+
+
+@pytest.mark.parametrize(
+    "api_mtls_endpoint", ["mtls.squid.clam.whelk", "mtls.squid.clam.whelk:443"]
+)
+@mock.patch("google.api_core.grpc_helpers.create_channel", autospec=True)
+def test_document_understanding_service_grpc_transport_channel_mtls_with_adc(
+    grpc_create_channel, api_mtls_endpoint
+):
+    # Check that if channel and client_cert_source are None, but api_mtls_endpoint
+    # is provided, then a mTLS channel will be created with SSL ADC.
+    mock_grpc_channel = mock.Mock()
+    grpc_create_channel.return_value = mock_grpc_channel
+
+    # Mock google.auth.transport.grpc.SslCredentials class.
+    mock_ssl_cred = mock.Mock()
+    with mock.patch.multiple(
+        "google.auth.transport.grpc.SslCredentials",
+        __init__=mock.Mock(return_value=None),
+        ssl_credentials=mock.PropertyMock(return_value=mock_ssl_cred),
+    ):
+        mock_cred = mock.Mock()
+        transport = transports.DocumentUnderstandingServiceGrpcTransport(
+            host="squid.clam.whelk",
+            credentials=mock_cred,
+            api_mtls_endpoint=api_mtls_endpoint,
+            client_cert_source=None,
+        )
+        grpc_create_channel.assert_called_once_with(
+            "mtls.squid.clam.whelk:443",
+            credentials=mock_cred,
+            ssl_credentials=mock_ssl_cred,
+            scopes=("https://www.googleapis.com/auth/cloud-platform",),
+        )
+        assert transport.grpc_channel == mock_grpc_channel
 
 
 def test_document_understanding_service_grpc_lro_client():