Virus-like particles were produced in insect cells containing either the L1 and L2 capsid proteins of bovine papillomavirus type 4 (BPV-4) or only the L1 protein. Both preparations of VLPs proved to be extremely effective prophylactic vaccines. Thirteen of 15 calves immunised with either L1-L2 VLPs or L1-VLPs were refractory to experimental challenge with high doses of BPV-4 and did not develop papillomas, while 9 of 10 control animals developed multiple oral papillomas. VLPs were not efficient as therapeutic vaccine in calves with established papillomas, although VLP-vaccinated animals appeared to undergo tumour regression more rapidly than nonvaccinated control animals. Antibody responses in VLP-vaccinated calves were associated with prevention of disease but not with regression of papillomas. Thus prophylactic VLP vaccination is effective in preventing disease in this model of mucosal papillomavirus infection. VLPs and native virus share at least some conformational epitopes, as shown by the cross-reactivity of their antibodies.