We previously showed that the reovirus type 3 hemagglutinin (HA) has distinct functional domains. For example, we identified one group of anti-HA monoclonal antibodies which only inhibited virus-mediated hemagglutination and another group which exclusively neutralized reovirus infectivity. Using competition radioimmunoassays, we now report that these functionally discrete domains on the reovirus type 3 HA correspond to discrete antigenic regions of the protein.