ABSTRACT: In our previous paper, we identified a novel sperm-coating antigen with molecular weight of approximately 12,000 daltons that is highly specific to sperm, seminal plasma, and milk from the plasma membrane fraction of human spermatozoa by using rabbit antiseminal plasma antiserum. In the present study, this 12,000-daltons component, termed gp12, has been investigated for its tissue distribution and antigenic stability. The largest amounts of the antigen are found in seminal plasma, although individual variation is rather high. In seminal plasma, the gp12 molecule presents in a large form with a molecular weight of approximately 50,000 daltons. Its antigenicity is stable when treated with acid, alkali, heat, and various protein denaturants.