Interoperability, in the electronic environment of healthcare enterprises, is required for the effective communication between the various Medical Information Systems (MISs). However, interoperability is a large and multidimensional issue. Its syntactic dimension is linked with the use of existing messaging standards. Their proprietary and specialized implementations have led to the establishment of advanced middleware platforms in order to support the communication between heterogeneous MISs. This paper approaches syntactic interoperability from the standardization point of view and proposes a Reference Implementation Model (RImM), for the middleware platforms. RImM specifies the functional entities of this platform, the end-to-end services and the internal common messages format. It also specifies common communication profiles and interfaces that allow real world events and information to be visibly handled and distributed among MISs without any interference in their structure.