A hemagglutination method employing tanned human O erythrocytes sensitized with purified Group A streptococcal polysaccharide has been developed to measure A polysaccharide antibody in antistreptococcal rabbit and human sera. The reaction of sensitized RBC with known Group A streptococcal rabbit antisera, inhibition of hemagglutination with A polysaccharide, and abolition of hemagglutination and hemagglutination-inhibition by exposure of polysaccharide antigen to A enzyme, which destroys the specific immunologic determinant of A carbohydrate, were utilized to demonstrate the specificity of this hemagglutinating system. The results of A polysaccharide antibody determinations on sera from normal persons, patients convalescent from streptococcal infection, and patients with non-suppurative streptococcal sequelae are presented.