Active immunization with the peptide segments rSMP-230 and YAL-198, corresponding to the hydrophilic extracellular domain of two human sperm antigens (rSMP-B and YWK-II, respectively), reduced fertility in female rats by different mechanisms. The anti-rSMP-230 antibody interferes with human and murine fertilization, and the anti-YAL-198 antibody blocks the development of mouse embryos. The authors examined in vitro at which stage the antibodies to rSMP-230 and YAL-198 were cytotoxic to murine embryos up to morula/blastocyst stage. Anti-rSMP-230 antibody was not cytotoxic to any stages. On the other hand, the anti-YAL-198 antibody arrested the growth of embryos at the 2-cell stage but not at more advanced developmental stages. When the anti-YAL-198 antibody was used, spotty staining was observed only on the surfaces of embryos that had arrested at the 2-cell stage. Unstained embryos, however, continued to develop normally. In contrast, the anti-rSMP-230 antibody stained murine sperm but failed to stain murine ova and embryos. The present results suggest that the human sperm components rSMP-B and YWK-II play important roles in sperm-egg interaction and early development of the embryo, respectively.