To determine whether sperm membrane components, rSMP-B and YWK-II, are suitable candidates as immunocontraceptives in humans, antifertility activities of the antibodies to the peptide fragments, rSMP-229 and rSMP-230 of rSMP-B and YAL-198 of YWK-II, were examined. In a previous report, anti-rSMP-230 antibody was shown to immobilise human sperm and to block human fertilisation, and the antigen (rSMP-230) to interact with antisperm antibodies found in sera of infertile women. Antibody to the second synthetic peptide, rSMP-229, corresponding to a different segment of rSMP-B, mimicked the biological activities of the anti-rSMP-230 antibody. Anti-YAL-198 antibody significantly, although weakly, inhibited human fertilisation. In the murine model, the anti-rSMP-B antibodies blocked in vitro fertilisation of mouse eggs but had no influence on embryo growth. Anti-YAL-198 antibody, however, arrested the growth of zygotes. In conclusion, rSMP-B, a human sperm protein, is a promising candidate in the development of an immunocontraceptive for human application. A second sperm protein, YWK-II, is effective as an antifertility immunogen in experimental animals.