A few days ago I had a real strange problem while using HttpClient in combination with ASP.NET Web API and integrated windows authentication (IIS Express). To keep it simple, I always got a 401 (Unauthorized) error returned from the localhost website, even though I was setting up the HttpClient correctly (see screenshots) and the HttpClient was sending the authorization header (negotiate).

January 25, 2013     ASP.NET MVC, Web API, Fiddler, 401 Unauthorized, Integrated Windows Authentication