Polysera raised in rabbits to bacterially derived fusion proteins and synthetic peptides of the L1 and L2 ORFs of HPV-6b and -16 were tested for cross-reactivity to laboratory-produced infectious HPV-11 virions. The polysera were analyzed in a series of five different immunological assays including immunoperoxidase staining of the koilocytotic nuclei in sections of formalin-fixed, paraffin-embedded as well as fresh frozen sections of HPV-11 experimental condylomas generated in the athymic nude mouse xenograft system, ELISA, Western blots, and neutralization of infectious HPV-11 virions. ELISA and Western blot assays were used to determine whether the polysera identified external or internal epitopes on HPV-11 virions, and whether there was cross-reactivity to BPV-1 or laboratory-produced CRPV virions. Seven of a total of 12 sera were positive for reactivity to HPV-11 in one or more assays, but none of the reactivity was directed to external epitopes on the intact virions as determined by ELISA. None of the L1 products generated group-specific antigen (GSA) antisera including a synthetic peptide spanning the GSA site. The combination of assays clearly demonstrated that apparent false positive and false negative reactivities of different antisera were obtained for each assay system tested. Thus, no single assay could be used reliably to determine the true antiviral reactivity of a given polysera.