To analyze the biological activities of the alphavirus glycoproteins, eight different monoclonal antibodies against the two glycoproteins of western equine encephalitis virus were isolated. Five of the eight monoclonal antibodies were shown to be specific for El and three for E2 protein by an enzyme-linked immunosorbent assay and by radioimmunoprecipitation. Three of the five anti-El and all of the anti-E2 monoclonal antibodies inhibited hemagglutination by purified virions. One anti-El and two anti-E2 monoclonal antibodies possessed high virus-neutralizing activity.