One of the basic markers for the differentia-tion of bacteria is the so-called Gram reaction. The gram-positive bacteria are distinguished by their ability to hold back the dye-iodine com-plex, whereas the gram-negative organisms are decolorized aftertreatment with alcohol. The mechanism of the Gram reaction has not yet been unraveled, but Salton (326) favors an involvement of the cell wall and has suggested a permeability difference between the cellwalls of gram-positive and gram-negative bacteria as the basis for the Gram reaction. This is in agreement with the findings that the positive or negative response to this reaction is reflected in the different ultrastructure of the cell wall. The cell wall of a gram-positive bacterium shows in profile one thick and more or less homogenous layer, whereas the profile of the cell wall of a gram-negative bacterium is remarkably com-plex and consists of several layers. A number of excellent monographs and reviews have re-cently appeared (see references 57, 89, 118, and 269 for more detailed information). The polymers making up the cell walls are chemically quite different in these two groups of bacteria. The gram negatives contain as major components lipopolysaccharide, lipoprotein, and relatively little peptidoglycan (less than 10% of the total cell wall) in their cell walls, whereas the walls of gram positives are mainly composed of peptidoglycan (usually 30-70% of the total cellwall), polysaccharides or teichoicacid (or both), or teichuronic acid. The peptidoglycan is the only cell wall po-lymer commonto both gram-negative and gram-positive bacteria. It has also been found among blue-green algae (96, 102, 142). Thus peptidoglycan is a cell wall component of all procaryotic organisms. There are only a few halophilic bacteria, such as Halobacterium ha-lobium (326, 363) and Micrococcus morrhuae (177), which lack peptidoglycan. The composi-tion and structure of the peptidoglycan seem to be rather constant among gram negatives, but there is great variation among gram positives. Numerous reviews have recently appeared on the structure and biosynthesis of the peptido-glycan (109, 117, 237, 279, 284, 333, 354, 381, 386, 403, 426). Since that time our knowledge about the diversity of the chemical structure of the peptidoglycan has increased. More-over, a high percentage of all known genera and of many different species of bacteria has been examined, and it now seems worthwhile to focus on the correlation of peptidoglycan structure and taxonomy. Therefore, in the first part of this review a classification of the known peptidoglycan typesbased on their mode of cross-linkage will be suggested. In the second part, the distribution of the various peptidoglycan types within the bacterial king-dom will be shown and their taxonomic signifi-cance will be evaluated. The following are abbreviations and uncom-mon amino acids and amino sugars used throughout the paper. Abbreviations: ATCC, American Type Culture Collection, Rockville, Md., USA; CCM, Czechoslovak Collection of Microorganisms, JE Purkyne University, Brno, Czechoslovakia; Kiel, Streptokokkenzen-trale im Institut fur Milchhygiene der Bundesanstalt fur Milchwirtschaft, Kiel, BRD; NCDO, National Collectionof Dairy Organisms, Reading, England; NCIB, National Collection of Industrial Bacteria, Aberdeen, Scot-land; NCPP, National Collection of Plant Pathogenic Bacteria, Harpenden, England; NCTC, National Collection of Type Cultures, London, England. Amino acids and amino sugars: GlcNH2 or G, glucosamine; Dab, diaminobutyric acid; m-Dpm, meso-diaminopimelic acid; Hsr, homoserine; HyDpm, hydroxy-diaminopimelic acid; Hyg, threo-3-hydroxy …