Three murine major histocompatibility complex (MHC) class II-restricted T cell determinants were identified in the major capsid protein L1 of human papillomavirus (HPV) type 16. Peptides derived from HPV-16 L1, which contain putative T cell epitopes located by a predictive algorithm, were synthesized and tested for lymphoproliferative activity by direct immunization, followed by in vitro assay of responses to peptides or recombinant HPV-16 L1. The MHC restriction of the stimulatory peptides was determined using blocking monoclonal antibodies against class II molecules. The responses, which were specific for the priming peptides alone, cross-reacted with recombinant L1 but not with analogous peptides derived from other HPV types.