Antigenic characteristics of nine mumps virus strains were determined by immunofluorescence and radioimmunoprecipitation assay (RIPA) using a collection of 44 monoclonal antibodies. These antibodies were directed against five different structural components of mumps virus, the haemagglutinin-neuraminidase (HN), fusion (F), matrix (M), phospho- (P) and nucleocapsid (NP) proteins. The nine mumps virus strains could be divided into two groups according to their antigenic characteristics. One group included two strains isolated more than a decade ago and the Jeryl Lynn vaccine strain. These three strains reacted with a wider range of monoclonal antibodies than the second group of six recently isolated strains of different geographical origin. In the F, M and P proteins variations were only found in single antigenic determinants. In the HN and NP components, RIPA revealed variations in three and seven determinants respectively. The Jeryl Lynn vaccine strain showed a unique lack of reaction with one anti-HN antibody clone in the RIPA.