Recent studies on the chemical composition of the bacterial cell wall indicate that the basic structural component, which is responsible for the rigidity of the wall, is similar in all bacteria examined (1). This component, designated as the cell wall mucopeptide, is composed of N-acetylglucosamine, N-acetylmuramic acid (3-O-lactyl-N-acetylglucosamine), and a limited number of amino acids. In the case of streptococci these two amino sugars and alanine, lysine, glycine, and glutamic acid have been identified in hydrolysates of trypsinized cell walls. However, this analysis of hydrolysates does not provide information on the arrangement of these substances in the chemical structure of the cell wall. The data reported here indicate that the two amino sugars and the four amino acids do in fact comprise a mucopeptide fraction. This substance is distinct from the other major cell wall component, the group-specific carbohydrate hapten, which contains rhamnose and N-acetylglucosamine.The work of Salton (2) has shown that the lysis of certain bacteria by lysozyme is accompanied by the release of the cell wall mucopeptide into soluble fragments. Lysis of streptococcal cell wall by phage-associated lysin is shown to be associated with a similar release of fragments of mucopeptide. These findings support the growing evidence which suggests that the specific linkage split by lysozyme and other cell wall-dissolving enzymes resides in the mucopeptide fraction.Materials and Methods Streptococcal Strains.--Group A streptococcal strains T12, D58, and $43/100, and Group A-variant strains T27A and K43 var. were used to prepare cell walls.Preparation of Call Wal/s.--The cell wails were prepared by the method of Salton and Home which employs a Mickle disintegrator (3). Differential centrifugation was used to separate the cell walls from the fine particulate material; and the walls were then treated with trypsin in phosphate-buffered saline, dialyzed against water, and finally lyophilized.