Thanks for the reply - but I think it was another problem. I checked the .NET configuration settings and by default the My_Computer_Zone covers the location of the assembly with the FullTrust permission set. The problem appears to be that the referenced assembly had a security attribute set and when viewing the properties of the file I saw the following message: This file came from another computer and might be blocked... I clicked the Unblock button, rebuilt and it worked. Thanks for the help.