The inhibitory activities of representative beta-lactam compounds, such as penicillins G and N, cephalosporins C and G, clavulanic acid, nocardicin A and thienamycin against Escherichia coli KN-126 and Bacillus megaterium KM peptidoglycan transpeptidases were studied. Their modes of action against E. coli are discussed on the basis of the results and the published binding data for penicillin binding proteins. The effects of modifications at position 3 and 7 of the cephalosporin and those at alpha-carbon of the benzyl side-chain of cephalosporin G and penicillin G were studied. The introduction of an amino group at this position in cephalosporin G together with the removal of an acetoxy group from the acetoxymethyl group at position 3 reduced the inhibitory activity against E. coli transpeptidase considerably. The activity was restored by the replacement of the methyl group at position 3 of cephalexin with chlorine. The restoration was accompanied by about 15-fold increase in the lytic activity of cephachlor against E. coli.