Recombinant human papillomavirus type 11 (HPV-11) virus-like particles (VLPs) were tested for their ability to induce the formation of neutralizing antibodies, and were also tested for serodiagnostic capabilities in an ELISA in comparison with HPV-11 whole virions. VLPs, purified by CsCl density gradient centrifugation from the cell-free supernatant of Ac11L1-infected Sf9 suspension cell cultures, were used to immunize rabbits and anti-VLP antibodies were tested in the athymic mouse model of HPV-11 infection. Pretreatment of infectious HPV-11 virions with the immune serum of VLP-treated animals caused a marked reduction of graft growth (P < 10(-4)) and viral gene expression (P < 10(-4)), similar to the effects obtained using whole virion postimmune serum, and consistent with immune neutralization. To assess the serodiagnostic capabilities of VLPs, a VLP ELISA was developed and used to analyse sera that were tested previously in an HPV-11 whole virion ELISA. Specific antibodies were detected in 49% of patients' sera (P = 2 x 10(-4)), and individual VLP seroreactivities correlated with those previously obtained using whole virions as the antigen (r = 0.87; P < 10(-6)). These results indicate that recombinant VLPs can be used to elicit a neutralizing antibody response, and can substitute faithfully for native virions in the development of HPV-serodiagnostic immunoassays.