Mouse monoclonal antibodies were generated against intact infectious BPV-1 virions by methods previously described (Christensen et al., 1990). ELISA was used to screen for reactivities to intact and/or disrupted BPV-1, CRPV and HPV-11 virions. Several hybridomas were initially selected that showed antibody reactivity by ELISA to both intact and disrupted BPV-1, to disrupted BPV-1 only, or to intact BPV-1 virions. One monoclonal antibody, designated B1.A1, which reacted only to intact BPV-1 was selected for virus neutralization analyses. ELISA demonstrated that this monoclonal antibody bound to intact BPV-1 virions, but not to intact CRPV, HPV-11 or to disrupted papillomavirus (PV) antigens. Strong neutralization of BPV-1-induced focus formation of mouse C127 cells by monoclonal B1.A1 was observed. The neutralization titer was equivalent to the neutralization titer obtained with a polyclonal rabbit anti-BPV-1 virion antisera, and directly correlated with antibody concentration as determined by ELISA. These results extend our previous analyses on the epitopes of infectious papillomaviruses as defined by monoclonal antibodies that identify neutralizing epitopes. The nature of these epitopes is such that maintenance of the quaternary structure of the infectious virions is necessary for preservation of the antigenicity of the neutralizing epitope.