Splenic lymphocytes from BALB/c mice immunized with Parainfluenza 1 virus (PIV-1), HA2 strain, were fused to the non secreting 653 and secreting P3 X 63Ag8 myeloma cell lines. Fusion products were screened for antibody synthesis using an enzyme-linked immunosorbent assay (ELISA) with purified PIV-1. Antibodies were characterized by their biologic specificity and immune fluorescence staining--6 monoclones (2HI and 4 non HI) were used to further analysis by cross reactivity panels using different PIV strains. None of the anti PIV1 monoclones showed any cross reactivity with PIV2 and PIV3 whereas with HI and non HI monoclones antigenic relationships were observed between viruses belonging to serotype 1. Monoclonal antibodies were also applied to the study of host cell related changes in PIV1 proteins.