It was reported that M1 monoclonal antibody (M1 mab) inhibited hamster sperm-egg fusion in a dose-dependent manner. The antigen was localized to the equatorial segment (ES) of the hamster sperm, and that after the acrosome reaction, it was exposed on the surface of the sperm plasma membrane overlying the ES. The M1 antigen therefore fulfilled the main requirements of a sperm component that might be involved in gamete fusion processes. Hence, it was necessary to fully characterized this sperm determinant in order to elucidate its role in fertilization and to assess its potential as contraceptive antigen. In this study, M1 mab was used to analyze the developmental expression of M1 antigen during spermatogenesis and during sperm maturation in the epididymal tract. Indirect immunofluorescent staining (IIF) assay revealed that the M1 antigen was detected only in the cross sections of seminiferous tubules containing spermatids and sperm. The specificity of M1 mab was examined also by IIF assay of cell suspensions prepared from various tissues of the hamster. The result showed that the M1 mab did not cross react with cells isolated from somatic tissues including lung, heart, kidney, spleen, brain and liver. Further detection by im-munoblotting also showed that the M1 antigen appeared to be sperm-specific as it was not detected in a range of somatic cells. It is therefore proposed that the M1 an-tigen which is synthesized only by the germ cells of the testis, also found to be ex-pressed at late spermatid stage and is a sperm-specific protein.