Blood group antigens A and B was demonstrated on human spermatozoa by a) conventional mixed agglutination test, ((b), immunofluorescense, and c) mixed agglutination performed with a mixture of fluorescein labelled and unlabelled erythrocytes as indicator cells for A or B specificity by fluorescence microscopy. Spermatozoa obtained from individuals with the blood group AB and who were secretors showed surface antigens with either A or with B specificity. The results by indirect immunofluorescence were in accordance with those of the two kinds of mixed agglutination. About 30-35% of spermatozoa showed fluorescence when the antibodies were directed against only one of the antigens of the heterozygous donor. With a mixture of antibodies corresponding to the two genes of the donor the percentage of the fluorescent spermatozoa was consistantly higher, about 50-55%. However, with a mixture of antiserum, no complete sum of the percentages was observed, compared with the monospecific antisera. Unreacted, "neutral" spermatozoa were always present in the samples. The limitations of the applied methods for a quantitative detection of antigens were discussed,