A protein agglutinogen common to the internationally recognized serotypes of Staphylococcus aureus is described. Sera of all human subjects studied, healthy or infected, possess a high and fairly uniform level of antibodies against this agglutinogen. The agglutinin is concentrated in commercial human γ-globulin and is deficient in the blood of agammaglobulinemic children. No immunizing agent has been found which will regularly increase agglutination titers in man to a significant degree, although some increase has been induced in rabbits. Serum absorbed with the pure protein agglutinogen shows decreased agglutination titer against all serotypes but is not deprived of its phagocytosis promoting power.