diff --git a/coreapi/transports/http.py b/coreapi/transports/http.py
index a548024..4c741b5 100644
--- a/coreapi/transports/http.py
+++ b/coreapi/transports/http.py
@@ -345,16 +345,22 @@ def __init__(self, credentials=None, headers=None, auth=None, session=None, requ
             session.cookies.set_policy(BlockAll())
 
         if credentials is not None:
-            warnings.warn(
-                "The 'credentials' argument is now deprecated in favor of 'auth'.",
-                DeprecationWarning
-            )
-            auth = DomainCredentials(credentials)
+            if auth is not None:
+                warnings.warn(
+                    "Ignoring 'credentials' argument in favor of 'auth'.",
+                    stacklevel=2
+                )
+            else:
+                warnings.warn(
+                    "The 'credentials' argument is now deprecated in favor of 'auth'.",
+                    DeprecationWarning, stacklevel=2
+                )
+                session.auth = DomainCredentials(credentials)
         if request_callback is not None or response_callback is not None:
             warnings.warn(
                 "The 'request_callback' and 'response_callback' arguments are now deprecated. "
                 "Use a custom 'session' instance instead.",
-                DeprecationWarning
+                DeprecationWarning, stacklevel=2
             )
             session.mount('https://', CallbackAdapter(request_callback, response_callback))
             session.mount('http://', CallbackAdapter(request_callback, response_callback))