S. W. 7 The d istribution of diamino acids in cell walls of bacterial species bears some relation to tax- onomy. The most widely distributed diamino acid is meso-diaminopime lic a cid which is present i n probably all Gram-negative species and in numerous other gen- era. L-lysine, also f airly common, is present in most Gram-positive c occi and in certain other species. Less frequent are DD or LL-diaminopimeli c, P-OH-diamino- pimelic, D or L ornithine, D or L diaminobutyric. The positions of these bifunctional amino acids in mucopep- tides (glycopeptides), the cross linked polymers of the walls, are described. Mucopeptides are divided i nto two types according to the site of termination of the cross-link from the D-alanine of an adjacent peptide chain. In type D, the site is the diamino acid which is located in the main peptide c hain; in type G (less com- mon) the site is the D-glutamic a cid, a nd the diamino acid is in the cross link. Other differentiating features of types D and G include t he optical configuration of the diamino acid, and the nature of the amino acid linking the peptide chain to the hexosamine backbone.