Using murine monoclonal antibodies (MAbs) to rubella virus haemagglutinin, five epitopes were identified in competitive ELISA binding assays: A, B, D and E by haemagglutination-inhibiting (HI) MAbs with no neutralizing (Nt) activity, and C by a MAb with neither activity. However, when HI and Nt activities were determined in the presence of anti-mouse immunoglobulins, epitopes A, B and D were defined by both HI and Nt MAbs, whereas epitopes C and E were identified by HI MAbs without Nt activity. A synergistic Nt activity, in the absence of anti-mouse immunoglobulins, was displayed by mixtures of antibodies of different epitope groups. Analysis of mixtures of MAb pairs each belonging to a different epitope class, showed that synergistic Nt activity was elicited primarily by the group A epitope, secondarily by groups B and D and only minimally by groups C and E.