Hybridomas secreting monoclonal antibodies to simian virus 5 (SV5) were obtained following immunization of mice with purified preparations of a human isolate (LN) of SV5. Immune precipitation studies showed that these monoclonal antibodies had specificities for the haemagglutinin-neuraminidase (HN), fusion (F), nucleo-, matrix and phospho- (P) proteins of SV5. By use of a radioimmune competition assay the monoclonal antibodies to the HN protein were assigned to four groups, members of which recognized different antigenic sites on the protein. All the anti-HN antibodies and the anti-F antibody neutralized virus infectivity. The 54 monoclonal antibodies obtained were used to determine whether there were antigenic differences between five human, two canine and one simian isolate of SV5. Although most of the monoclonal antibodies reacted with all isolates, a few did reveal antigenic differences in the HN, F and P proteins. Furthermore, analysis by SDS-PAGE showed that while the electrophoretic mobilities of most of the virus polypeptides of these isolates were similar some differences could be detected. In particular the P protein showed the most marked mobility differences between the human, canine and simian isolates. Slight differences in the mobility of the F1 glycoprotein could also be visualized.