Three monoclonal antibodies, termed 4E10, 1E11:10, and 2D9:1, were generated against rubella virus. Immunoblot analysis with purified authentic rubella virus or recombinant baculovirus-expressed rubella virus structural proteins E1, E2, and C demonstrated that they were directed against the E1 envelope glycoprotein of the rubella virus particle. By using the yeast Ty virus-like particle system, it was possible to map the binding site of 1E11:10 within amino acids 236 to 286 of the E1 protein and the binding sites of 2D9:1 and 4E10 outside this region. Immunoaffinity purification with these monoclonal antibodies made it evident that they are useful for obtaining large quantities of pure baculovirus-expressed rubella virus envelope protein E1. The diagnostic potential of this immunoaffinity-purified recombinant rubella virus E1 protein compared with that of authentic rubella virus is demonstrated.