The reaction between staphylococcal protein A and human γG globulin, which is known to be mediated by structures on the Fc part of the heavy chains, was studied qualitatively using 68 isolated γG myeloma globulins. Twenty-one myeloma globulins of subgroups γG-1, γG-2 and γG-4 gave a precipitin reaction with protein A. The 35 remaining meyloma globulins of these subgroups all inhibited this reaction, indicating the presence of protein A-reactive sites. Twelve γG-3 myeloma globulins did not precipitate or inhibit the precipitation; two showed slight inhibition that could be attributed to background γG. It thus appears that anti-protein A reactivity is confined to γG-1, γG-2 and γG-4 molecules.