SUMMARY Histocompatibility antigens HL-A1, 2, 5, and 7 can be detected on leukocytes by a simplified complement-fixation method, already applied on platelets. The main problem was the late manifestation of antigenic activity on the cellular surface: it was shown that treatment of leukocytes with ammonium oxalate can uncover this reactivity. The number of leukocytes required was 2.5 X 106 cells/test; a complete typing (with a battery of about 20 antisera) may thus be carried out with 20 ml of blood. A panel of 100 persons was tested using this method with seven anti-HL-A sera of known specificity; the correlation with the HL-A types determined by the lymphocytotoxicity and platelet complement-fixation techniques has been highly significant. The minor discrepancies are discussed