The effects in vitro of D-amino acids or glycine on the formation of wall-bound peptidoglycan were studied with wall membrane enzyme preparations from Gaffkya homari. These amino acids inhibited the incorporation of nascent peptidoglycan into the preformed polymer (e.g. ID50 values for D-alanine, D-leucine, and glycine = 5.6 mmol/l, 1.3 mmol/l, and 11 mmol/l, respectively). The inhibition was accompanied by an incorporation of the inhibitor into position 4 of the peptide subunit Ala1-DGlu2(Lys3-DAla4), where the indices refer to the position of an amino acid residue within the peptide subunit. It is suggested that the reaction is catalyzed by an LD-carboxypeptidase. Therefore, this enzyme has also D-amino acid exchange activity. At inhibitory concentration fewer tripeptide subunits were formed in the nascent peptidoglycan in favour of the formation of tetrapeptide subunits bearing the inhibitor at the C termini. The tripeptide subunits are assumed to be necessary in order that nascent peptidoglycan is utilized as substrate in the transpeptidation reaction. Thus an essential role of the LD-carboxypeptidase is indicated.