The phagocytosis of complexes between staphylococcal protein A and 125I-labelled IgG is enhanced by fresh human serum and to a somewhat lesser extent by heat-inactivated serum. Opsonizing activity is detected in two distinct peaks by Sephadex G-200 column chromatography. One peak corresponds to the excluded material and the other to the ascending limb of the second protein peak. The active material in serum is retained on a protein A-Sepharose but not on an IgG-Sepharose column. Active material eluted from a protein A-Sepharose column and run on a Sephadex G-200 column again shows two activity peaks corresponding to those obtained on chromatography of whole serum. The first peak contains mainly IgA nd IgM but very little total protein material. The second peak contains mainly IgG but also IgA. Purified normal IgA, but not one myeloma IgA preparation tested, has the capacity to enhance phagocytosis of protein A-IgG complexes. Possible examinations for the opsonin activity are discussed.