The cell wall peptidoglycans were isolated from Clostridium botulinum and some other species of the genus Clostridium by hot formamide extraction and their quantitative chemical composition and antigenic properties were determined.The peptidoglycan of C. botulinum type E was found to be a diaminopimelic acid (DAP)-containing type composed of glucosamine, muramic acid, glutamic acid, alanine and DAP in the molar ratio of 0.76 :0.78 :1.00:1.88 :0.81. All other types of C. botulinum and Clostridium sporogenes also belonged to the same peptidoglycan type. The peptidoglycans of Clostridium bfermentans and Clostridium histolyticum contained DAP but they differed from those of C. botulinum in the molar ratio of alanine to glutamic acid. The peptidoglycan of Clostridium perfringens was composed of glutamic acid, alanine, DAP and glycine in the molar ratio of 1.00 :1.64 :0.94 :0.90. On the other hand, the peptidoglycan of Clostridium septicum was found to contain lysine instead of DAP and the molar ratio was 1.00:1.41 :0.96 for glutamic acid, alanine and lysine. In spite of the difference in amino acid composition of peptidoglycans among the clostridia, the quantitative precipitin test demonstrated that antiserum against C. botulinum type E peptidoglycan cross-reacted with the peptidoglycans from other clostridia as well as various types of C. botulinum.It is well known that the cell walls of grampositive bacteria are composed mainly of peptidoglycan, polysaccharides, teichoic acid or teichuronic acid. The peptidoglycan, which is the only cell wall heteropolymer common to almost all bacteria, is composed of repeating units of N-acetylglucosamine and N-acetylmuramic acid in which the Dlactate moiety of each muramic acid residue is substituted by a short peptide chain consisting of alternating L-and D-amino acid moieties, and the peptide chain is further connected to another adjacent peptide by various modes of cross-linkages [4,16,25].The chemical constituents of the cell walls of various species of the genus Clostridium were qualitatively determined by Cummins and colleague [2, 3] and these results were used for classification of this genus [19]. Although there are a few studies on the quantitative chemical analyses of peptidoglycans of some clostridia [12,13,15]