that's almost always a problem when you try to call EJBs in application servers running a different version of the same software. You may be able to deploy the clientside archives from JBoss 3 into your ear file that you deploy on JBoss 4, but mind that this could lead to problems trying to access resources that reside inside the JBoss 4 server (when we tried calling EJBs on a JBoss 4 server from JBoss 3 that way things went horribly wrong and we ended up using a SOAP layer to communicate between the two). View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4026940#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...