The development of a contraceptive vaccine based on a gamete-specific antigen requires knowledge of the ability of the antigen to elicit an immune response that inhibits fertilization. A well-defined immune response, as elicited by a synthetic peptide comprising a dominant B-cell epitope coupled to a common promiscuous T-cell epitope, might be preferable. In this study, the immunodominant B-cell epitope of sperm antigen Sp17 has been identified and synthesized as a chimeric peptide with the promiscuous T-cell epitope bovine RNase[94-104] at the N terminal. Immunization of female BALB/c mice with this peptide induced a dose-dependent reduction in fertility. Although antibodies to recombinant and native Sp17 were elicited in these mice, there was no strict correlation between the level of these antibodies and the reduction in fertility. Moreover, the induction of infertility was strain-specific since no effect on fertility could be induced in B6AF1 mice. To understand the mechanism behind this apparent strain-specific infertility induction, a more extended study on both the humoral and the cellular immune response to the chimeric peptide was performed. The antigen-specific T-cell response and the levels of antigen-specific cytokines are the major factors that affect fertility outcome.