Summary The size and antigenic relationships among structural proteins (VPs) of canine parvovirus (CPV), feline parvovirus (FPV), porcine parvovirus (PPV), minute virus of mice (MVM) and bovine parvovirus (BPV) were determined by SDS-PAGE of radiolabelled, purified virus and immunoprecipitated viral proteins. Mature virions of CPV, FPV, PPV and MVM were composed of three VPs designated VP1, VP2 and VP3. The corresponding proteins of each virus were similar in molecular weight [79000 to 82500 (VP1), 65000 to 66000 (VP2), 62000 to 63500 (VP3)]. Additional similarities among VPs were indicated by antigenic relationships which included precipitation of VPs of CPV, FPV and PPV by both homologous antisera and antisera raised to each of the other two viruses and by precipitation of VPs of MVM by cat anti-FPV sera. A non-structural protein identified in lysates of cells infected with FPV and CPV was precipitated by cat anti-FPV and dog anti-CPV sera only. Mature virions of BPV were composed of four VPs [74500 (VP1), 67000 (VP2), 60000 (VP3), 57500 (VP4)] which were antigenically unrelated to those of the other parvoviruses tested. However, the possibility that swine are sometimes infected with a virus which is antigenically related to BPV was suggested by the finding that sera from conventionally raised swine, irrespective of their serological status for PPV, precipitated VPs of BPV, whereas neither pre-exposure sera nor anti-PPV sera from gnotobiotic pigs did so.