The structure and biosynthesis of the cell wall of Stapkylococcus aureus, strain Copenhagen, have been investigated previously (cf. reference 1). Studies of the immunological specificity of this strain were undertaken for several reasons. The chemical nature of the surface structure (the cell wall) of this organism was under investigation and had been partially elucidated. If, as seemed likely, the ceil wall contained immunological determinants, the chemical nature of structures responsible for this activity could be defined. Relatively little is known about the chemistry of the antigens ha staphylococci or their relationship to such biological phenomena as immunity, allergy, plmgocytosis, and intracellular lysis. Moreover, it seemed likely that immunological methods would prove useful in further studies of the biosynthesis of the cell wall of S. aureus. These have been the stimuli to the present work.Rabbit anfisera were prepared against whole organisms of S. aureus, strain Copenhagen, and were found to agglutinate cell walls prepared from this species. IIspten inhibition of this agglutination has led to the conclusion that the teichoic acid (eft reference 2) in the cell wall (a polymer of ribitol phosphate which contains glycosidicaUy linked N-acetylglucosamine and esterified D-alanine) contains a serologicaily active group. Among the earliest studies of the antigens in S. aureus strains were those of Julianelle and associates (3, 4). They isolated a substance termed polysaccharide A 25 years ago and showed that it was active both in precipitin tests with rabbit antisera and in inducing wheal and erythema in humans. A similar, but distinct substance, termed polysaccharide B, was isolated from Staphylococcus aIbus strains. In retrospect the described properties, as well as the method of isolation, of polysaccharides A and B indicate that these compounds were teichoic adds. In recent years, immunological activity of the polyglycerophosphate (which belongs to the family of teichoic acids, i.e. polyol phosphate polymers) found in several Gram-positive species has been demonstrated by McCarty (5).