Coagulase-positive staphylococci produce discrete compact colonies when grown in a semisolid medium containing rabbit serum or plasma. In contrast, the same organisms produce elongated diffuse colonies when grown in the basal medium. Coagulase-negative staphylococci, on the other hand, produce diffuse colonies in both media, but were found to produce compact colonies when the basal medium was enriched with homologous antibody-containing serum.Sixty-three strains were studied, 56 of which were coagulase positive. The strains were from different sources—human, canine, bovine, and equine. A number of strains did not conform to the normal pattern. Attempts were made to establish the relationship between agglutinin titers and colony-compacting antibodies.