When adding the endpoint address of a service into the Microsoft WCF Test Client, I receive the error message included below. The latest versions of svcutil.exe and wcftestclient.exe were copied to the Windows 2008 Server on which a .NET application is hosted, which in turn is hosting the WCF service I am attempting to test. The application targets .NET Framework 4. I am able to navigate to the service enpoint using the ?wsdl syntax in order to view the service metadata using internet explorer. Any assistance will be appreciated. Thank you. Error Message: Error: Cannot load reference assembly 'C:\Windows\Microsoft.Net\assembly\GAC_32\System.Data\v4.0_220.127.116.11__b77a5c561934e089\System.Data.dll' Cannot load file C:\Windows\Microsoft.Net\assembly\GAC_32\System.Data\v4.0_18.104.22.168__b77a5c561934e089\System.Data.dll as an Assembly. Check the FusionLogs for more Information. Could not load file or assembly 'file:///C:\Windows\Microsoft.Net\assembly\GAC_32\System.Data\v4.0_22.214.171.124__b77a5c561934e089\System.Data.dll' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.