Monoclonal antibodies (MAbs) were obtained using a purified preparation of the RBOK strain of a rinderpest vaccine virus. The cytoplasmic immunofluorescent staining test showed that these clones had specificity for the nucleoprotein (N) of the virus. Six clones which immunoprecipitated the N protein corroborated these results. Thirteen anti-N MAbs were used to compare geographically widespread rinderpest viruses (RPV) and peste des petits ruminants viruses (PPRV) to two other morbilliviruses, measles (MV) and canine distemper (CDV). The N protein antigen profiles of the 23 isolates determined by immunofluorescent staining and enzyme linked immunosorbent assay (ELISA) on infected cells enabled us to classify the strains into groups. A differential identification of the morbilliviruses can be made using one MAb or associations of the MAbs. The potential to distinguish between RPV and PPRV and between virulent and avirulent strains of rinderpest is of primary interest.