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Salton & Home (1951) and McCarty (1952a, b) isolated insoluble material from disintegrated Gram-positive bacterial cells, which was shown by electron-microscopic examination to have retained the form of the cells from which it had been isolated, and both sets of authors concluded that they had isolated the cell walls. The prooedure involved disintegrating the cells by s a thick suspension with fine glass beads, separating the insoluble walls from unbroken cells and soluble cell contents by differential centrifuging and then repeatedly washing the wall material with water or sodium chloride solution. Enzyme preparations which would lyse the living organisms also lysed cell walls (McCarty, 1952a; Salton, 1952, 1956). Subsequently a somewhat different method was described (Cummins & Harris, 1956) for preparing wall material. This method replaced the repeated washing procedure of Salton & Home (1951) by treatment with the enzymes trypsin, ribonuclease and pepsin. Qualitative chemical examination (Salton, 1953; Cummins & Harris, 1956) of acid hydrolysates of wall material isolated by either method showed that it had a rather simple composition. When obtained from most species it yielded four or five amino acids, some in the Dconfiguration (Ikawa & Snell, 1956; Snell, Radin & Ikawa, 1955), one or two hexoses and two hexosamines. It is proposed to call the material mucopeptide. The two hexosamines usually present are glucosamine and its 3-0-x-carboxyethyl derivative. The latter was first recognized as an unknown amino sugar universally present in mucopeptide by Cumri & Harris (1956), and was more exaotly characterized by Strange & Dark (1956); it has since been synthesized by Kent (1958) (see also Strange & Kent, 1959). Although the earlier authors thought the material we propose to call mucopeptide constituted the whole cell wall of the organism, later work has somewhat complicated this picture. For example, Baddiley, Buchanan & Carss (1958) and Armstrong, Baddiley, Buchanan & Carss (1958) have shown that a polyribitol phosphate with combined amino acids in it can be extracted from cell-wall preparations from some species. The exact inter-relationship of the mucopeptide and these other substanees is not clear. It seems likely, however, that the mucopeptide does not represent the whole cell wall in some species. Nevertheless, observations such as those of Park (1958) and Mandelm & Rogers (1959) show that antibiotics such as penicillin and bacitracin inhibit its synthesis, and other strongly suggestive observations (Park & Johnson, 1949; Park, 1952a, b; Park & Strominger, 1957) have pointed to the importance of the inhibition of mucopeptide synthesis as a mech nism of action of some antibiotics. Apart from investigations of the action of antibiotics, the mucopeptides themselves have great intrinsic interest in that they represent examples of substances containing only a limited nuimber of both sugars and amino acids, yet, apparently, bearing considerable immunological specificity (Hundley, Ing & Krauss, 1956). Elucidation of their chemistry may help towards a further understanding of the more complicated mucoproteins. A preliminary account of some of this work has already appeared (Perkins & Rogers, 1958).

The products of the partial acid hydrolysis of the mucopeptide from cell walls of Micrococcus lysodeikticus