This paper outlines how Multi-Agent Systems can access Web Services. In particular a Multi-Agent based medical therapy proposition system is introduced. For fulfilling its tasks it has to access Web Services that are providing therapy information. The proposed solution uses a mediator that acts as an interface between the Multi-Agent System and the used Web Services. First the mediator transforms Agent Communication Language (ACL) messages from agents within the system conforming to SOAP1 specifications and accesses appropriate Web Services. The result is translated back to ACL and is consumed by the requesting agents. Furthermore asynchrony patterns to accelerate the system and security services are supplied by the mediator.