Staphylococcal agglutinogen was extracted from undigested cell walls and purified through carboxymethyl (CM)-cellulose column chroma tography and molecular sieving by Sephadex G-75. The sedimentation patterns of ultracentrifugation showed a high degree of homogeneity of the purified agglutinogen. The sedimentation constant is 1.6 S, the diffusion constant is 11.6 × 10-7 cm2/sec, and the molecular weight is 13,200. It showed single line precipitation reactions with human and rabbit antisera in agar gel diffusion and immunoelectrophoresis. From the nitrogen content, ultraviolet absorption, and sensitivity against digestion by proteolytic enzymes, it was shown that the agglutinogen is a protein. Jensen's antigen A contains the agglutinogen.