Six different genotypes of mumps virus, A, C, D, G, H and I, genotyped on the basis of the small hydrophobic protein gene sequence, were subjected to antigenic comparison. Monoclonal antibodies directed against the haemagglutinin-neuraminidase protein of the SBL-1 strain of genotype A were used in immunofluorescence tests with different mumps virus strains. In addition, the six virus genotypes were compared by cross-neutralization tests with human post-vaccination sera after vaccination with the Jeryl Lynn (JL) strain of mumps virus and with rabbit hyperimmune sera directed against the A or D genotypes of mumps virus. Genotypes C, D, G, H and I could not be antigenically separated. In contrast, three different virus strains of genotype A, SBL-1, JL and Kilham, were distinct and were found to represent three different serotypes within the A genotype of mumps virus. Vaccination of Swedish children with the JL strain of mumps virus resulted in clearly lower neutralization titres against the SBL-1 strain, which is endemic in Sweden, compared to the homologous vaccine titres.