Prostate acid phosphatase (PAcP) was characterized as one of carrier proteins for ABH blood group antigens in semen. The diluted semen was sensitized with anti-PAcP. The resulting immunocomplex was subjected to SDS-PAGE after being washed with saline. SDS-PAGE patterns of the immunocomplex consisted of four bands. After electroblotting from gel to nitrocellulose filter, immunostaining using anti-PAcP or monoclonal anti-blood group antibodies was performed. The result showed that the protein of 50 kD was the subunit of PAcP and the band of 94 kD was identified with blood group antigens respectively. Other two bands were rabbit IgG against PAcP and its fragment. When semen was treated with the chemicals such as papain, SDS, 2-mercaptoethanol and dithiothreitol, the ABH of antigen which was captured by anti-PAcP were considerably reduced. The reduction of ABH activity by these treatments indicated that ABH antigens were located on PAcP in semen which was inactivated by the treatment. Therefore, PAcP was one of proteins carrying ABH epitopes in semen in addition to alpha 2-seminoglycoprotein.