The incidence and significance of antisperm antibodies in different groups of men were evaluated by a modified enzyme-linked immunosorbent assay. In serum, 4.0% of dermatologic patients (n = 223), 9.6% of andrologic patients (n = 178), and 28.6% of homosexual men (n = 42) were positive for IgG and/or IgM antibodies. In seminal fluids, 7.3% of the andrologic patients had IgA (and IgG) antibodies to spermatozoa. Only 1 of 29 positive men had antibodies both in serum and in seminal fluid. No correlation between antisperm antibodies and IgG/IgM concentrations was found in serum, whereas in seminal plasma men with antisperm antibodies showed higher IgG/IgA concentrations than men without (IgA, 3.2 versus 1.7 mg/dl; IgG, 9.8 versus 6.3 mg/dl). It is concluded that there is a high incidence of antisperm antibodies among homosexual men, probably because of contact of spermatozoa with the immune system by passive anal intercourse. There is little correlation between antisperm antibodies in serum and seminal plasma of infertile men because of a lack of relevant antibody transfer from the serum and the formation of local antibodies in seminal plasma. Antisperm antibodies in seminal fluid are associated with elevated local IgG and IgA concentrations.