Monoclonal antibody-based competitive ELISA (C-ELISA) have been used for the specific measurement of antibodies to both rinderpest and peste des petits ruminants (PPR) viruses in cattle, sheep and goats. Examination of serum samples from sheep and goats in Gambia, before and after vaccination with rinderpest vaccine, suggested that antibodies to PPR virus could prevent an immune response to the rinderpest vaccine. Cattle sera from Nigeria and Ghana showed a high prevalence of antibody against PPR virus which may explain the difficulty experienced in some countries in achieving high post-vaccination immunity levels against rinderpest. Because antibodies against PPR virus are both cross-neutralizing and cross-protective against rinderpest virus further vaccination in the presence of antibodies against PPR virus may be a waste of national resources. This paper presents serological evidence for the transmission of PPR virus from sheep and goats to cattle and highlights the need to include PPR serology in the sero-monitoring programme to give a better indication of national herd immunity.