The binding of latamoxef (moxalactam) and of a decarboxylated derivative to Escherichia coli and Pseudomonas aeruginosa penicillin-binding proteins (PBPs) was measured by competition experiments with 125I-radiolabelled penicillin X. Latamoxef and the decarboxylated derivative were highly bound to most of the PBPs, with the exception of PBP-2. As the two compounds possess a phenolic side-chain, they also could be radiolabelled with 125I. The proteins thus labelled by these derivatives were qualitatively the same as those labelled by 125I-penicillin X, except for PBP-2 which was not labelled by the iodo derivatives of latamoxef and its decarboxylated derivative, and PBP-1c (in E. coli) which is labelled only poorly by the radioactive penicillin. No important difference between latamoxef and its decarboxylated derivative was found, and the same observation was made for penicillin G and carbenicillin. Thus, it was concluded that the carboxylic group of latamoxef does not play an important role in affinity for the targets.