ABSTRACT: This is the first report of induction of high titer mouse-antihuman spermatozoal antibodies using a spermatozoal antigen incorporated in liposomes. The peptide antigens that were incorporated into pure phosphatidylcholine liposomes are known to react with the naturally occurring antibodies in sera of sterile patients. Electron microscopy and the complement-dependent glucose release tests indicated that the peptides were entrapped in the aqueous phase between the lipid bilayers in the liposome vehicles. This is in contrast to most other liposomal antigens that are expressed on the outer lipid surface. In view of the nontoxic nature of the liposomes these results could be of some interest in immunological fertility regulation trials with appropriate antigens in an allogeneic system.