Antibody and complement fixation by viable and nonviable spermatozoa were studied by means of immunofluorescence and a hemolytic assay (CH-50). Spermatozoa were incubated in sera from fertile male and female donors and in peritoneal fluid from fertile women undergoing laparoscope Nonviable spermatozoa were able to bind antibody or complement from sera and peritoneal fluid. There was no evidence of antibody or complement fixation by viable spermatozoa. The antibodies present in the serum that bind to spermatozoa belong to the IgG and IgM class; in peritoneal fluid, only IgG could be found. Complement fixation occurred via the classical (antibody-mediated) and alternative pathway. Viable spermatozoa possess antigenic properties different from nonviable spermatozoa. The lack of immunological reaction of women to viable spermatozoa and a possible mechanism for immunological infertility is considered.