The inhibition effects of SpA binding to anaphylactic sensitization activities of antibody molecules in guinea pigs were quantitatively investigated using isolated guinea pig subclass antibodies, IgG1 and lgG2, and rabbit antibody lgG against BSA. It was found that the anaphylactic sensitization activities of the antibodies both in PCA and systemic reactions were completely inhibited by the binding of SpA with these antibodies, and that the concentration of SpA required to inhibit the sensitization activities was almost the same as those of the immunoglobulins. The possibility that the inhibition effects with SpA binding are related to the blockage of interaction sites of the antibody molecules to Fc receptors on their target cells is discussed.