A passive hemagglutination technique employing isolated γ-globulins and myeloma proteins of known genetic types coated onto human group O cells by CrCl3 method has been developed. Cells thus coated have been successfully used for Gm and Inv typing of human sera. The degree of discrimination between inhibiting and noninhibiting sera is as great as with the conventional method using cells coated with incomplete anti-Rh.