Calves, free of antibodies to bovine papillomaviruses (BPV), were reared in isolation. One was infected with BPV-2, developed tumours and was resistant to homologous reinfection. Groups of calves were infected with BPV-2, BPV-5 or BPV-6; they all developed and subsequently rejected type-specific tumours. They were then infected with BPV-4; they were not immune and oral papillomas were induced. Groups of animals were vaccinated by intramuscular preparations of purified BPV-4 and BPV-6 and were challenged with homologous virus; all were immune to reinfection. An earlier experiment had shown this to be true for BPV-2. Two calves, immune to BPV-6, were not immune to BPV-1. These experiments, although they do not cover all the possibilities of reciprocal immunisation and challenge, indicate that prophylactic immunity to a range of papillomaviruses is type-specific. This is the first clear demonstration of this phenomenon in the papillomavirus group.