diff --git a/starlette/middleware/wsgi.py b/starlette/middleware/wsgi.py
index f0c69c8d..fbc8411b 100644
--- a/starlette/middleware/wsgi.py
+++ b/starlette/middleware/wsgi.py
@@ -120,7 +120,7 @@ class WSGIResponder:
             status_code_string, _ = status.split(" ", 1)
             status_code = int(status_code_string)
             headers = [
-                (name.strip().encode("ascii"), value.strip().encode("ascii"))
+                (name.strip().encode("ascii").lower(), value.strip().encode("ascii"))
                 for name, value in response_headers
             ]
             self.send_queue.append(