The immunological relationships between mumps virus and parainfluenza viruses were investigated with 74, 78 and 80 previously developed monoclonal antibodies directed against five major structural proteins of mumps virus, Sendai virus (a murine parainfluenza type 1 virus) and parainfluenza type 3 virus. These monoclonal antibodies were reacted with the three viruses, with parainfluenza type 2 virus and with Newcastle disease virus (NDV) in ELISA and immunofluorescence (IF) tests. In addition, immunoprecipitation tests with [35S]methionine-labelled extracellular virions were carried out with cross-reacting monoclonal antibodies. None of all 232 monoclonal antibodies against the three viruses cross-reacted with either parainfluenza type 2 virus or NDV in ELISA and IF tests. In the collection of 74 mumps virus monoclonal antibodies, three directed against the nucleocapsid (NP) protein, polymerase protein, and fusion protein cross-reacted with Sendai virus. Two Sendai virus monoclonal antibodies directed against two different epitopes of the haemagglutinin-neuraminidase (HN) protein cross-reacted with parainfluenza type 3 virus. Six other Sendai virus monoclonal antibodies directed against four different epitopes of the HN protein and one directed against the NP protein cross-reacted with mumps virus. Eight out of 80 monoclonal antibodies directed against parainfluenza type 3 virus cross-reacted with Sendai virus. One was directed against the HN protein, four were directed against a minimum of two epitopes of the matrix protein and three were directed against three different epitopes of the NP protein. The different cross-reactions found show that Sendai virus is antigenically related to both mumps virus and parainfluenza type 3 virus. In contrast, no antigenic relationship could be demonstrated between mumps virus and parainfluenza type 3 virus.