The proliferative responses to synthetic peptides by lymphocytes derived from rinderpest virus (RPV)-infected cattle, the natural host for RPV, were assayed by determining [3H]thymidine incorporation into the DNA. In eight out of twelve cattle tested, significant responses were detected to peptides representing amino acids 452-501 in the C-terminal hypervariable region of the virus nucleocapsid (N) protein. It appears that helper T-cell epitope(s) for cattle which can be broadly recognized within an MHC diverse population, exists in this region of the protein.