To study cellular immunity towards bovine papillomavirus (BPV), calves were infected intradermally with BPV-1, and the cellular immune response was measured by the lymphocyte proliferation assay. Peripheral blood lymphocytes were obtained which in control experiments were highly reactive towards mitogen stimulation. Different batches of BPV-1 were prepared by the use of density gradients. In the first set of experiments, a nonspecific mitogenic effect of the virus preparation was observed. This effect was obviously caused by soluble molecules contained in the virion-free supernatant obtained after ultracentrifugation of the viruses. Subsequently, we obtained highly purified virions which did not share the nonspecific mitogenic effect. The data obtained with these virions showed no responses of untreated control animals but a short-lived in vitro lymphoproliferative response 2 to 3 weeks after infection. This response then disappeared and was followed by a period of non-responsiveness which lasted as long as we tested the animals.