SummaryThis study localized antigenic determinants recognized by a mouse anti‐human sperm monoclonal antibody TüS10 immunocytochemically and immunoelectron microscopically in the rat sperm recovered from the caput and cauda epididymidis. Immunocytochemistry showed that the antibody bound specifically to the plasma membrane overlying the principal piece of membrane‐intact sperm from the caput and cauda epididymidis. Demembranation by Triton X‐100 significantly decreased the affinity of the monoclonal antibody TüS10 to the caput sperm but did not obviously change that to the cauda sperm. Immunoelectron microscopy with biotinstreptavidin peroxidase complex pre‐embedding method confirmed the localization of the antigenic determinants over the cell surface of the principal piece of the membrane‐intact spermatozoa from the caput and cauda epididymidis. The demembranated sperm from the caput epididymidis showed no intracellular labelling, while those from the cauda displayed labelling on their external surface of the fibrous sheath. Using monoclonal antibody TüS10 as a probe, we detected different distribution patterns of the antigenic determinants between the spermatozoa in the caput and cauda epididymidis. These results suggest that spermatozoa mature with immunologically detectable changes in the fibrous sheath during their epidid