Spontaneous sperm agglutination was observed in the ejaculate of a 39-year-old male (patient Z). Using an enzyme-linked immunosorbent assay (ELISA) with spermatozoa fixed to wells of a microtitre plate it was determined that IgA was associated with patient Z's spermatozoa but not with a pooled sample of spermatozoa from fertile men Z's seminal fluid agglutinated human spermatozoa (titre 1:16). In contrast, no sperm-agglutinating or immobilizing activity was present in the patient's serum. Agglutination was prevented by preincubation of the seminal fluid with anti-IgA antibody but not with anti-IgA antibody. IgG from Z's seminal fluid readily bound to human spermatozoa, as determined by the ELISA assay. IgA from a pool of seminal fluids from fertile men or from 18 human sera bound to spermatozoa to a much lesser extent. By sucrose-gradient velocity centrifugation it was demonstrated that the anti-sperm IgA in patient Z's seminal fluid was 11S. Finally, antibody to secretory component removed the anti-sperm IgA from Z's seminal fluid. These data provide evidence that secretory IgA with anti-sperm activity is produced in the male genital tract. Furthermore, a new methodology is presented for the determination of antibodies to spermatozoa.