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,

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... h 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.