Technically, all you need to be able to use .Net with MQSeries is the ampmdnet.dll (that and the proper security access the the queues and queuemanagers...but that another story). The WebSphere client is just the "human" interface to MQSeries. Kinda like the way SQL Server Management Studio is the "human" interface to SQL Server. From what I've been able to find out though, the most recent version of WebSphere still uses v1.0.0.3 of amqmdnet.dll, which should be what you have with your 5.3 version.