We believe that successful co-operation between multi-agent systems providing services in a specific domain can be realized by constructing an infrastructure that supports the semantic interoperability between them. In this paper(1), we introduce a conceptual architecture for the semantic interoperability of multi-agent systems in the large-scale. The most important element of the proposed conceptual architecture is the federation. A federation established for a particular. domain specifies the common characteristics of multi-agent systems in that domain. Hence, a special ontology is required at the federation level for defining the common characteristics of each multi-agent system in a particular domain as its own meta-knowledge. This meta-knowledge is then used in the semantic discovery of the multi-agent systems with which to cooperate. Since different multi-agent systems may use different ontologies, an ontology translation service is also defined at the federation level.