This paper discusses semantic interoperability issues in agentbased E-commerce systems. The literature reports various techniques to enable agents to understand the meanings of the messages exchanged. We will argue how these different techniques can be combined in one agent communication protocol to obtain the best of each world. The resulting communication protocol enables agents to sufficiently understand each other to participate in successful collaboration. Categories and Subject Descriptors I.2.11 [Artificial Intelligence]: Distributed Artificial Intelligence--multiagent systems, coherence and coordination; D.2.12 [Software Engineering]: Interoperability--data mapping General Terms Standardization, Languages Keywords Electronic Commerce, Semantic Interoperability, Ontologies