beta-lactamase induction in Enterobacter cloacae, which is linked to peptidoglycan recycling, was investigated with use of high-performance liquid chromatography of cell wall fragments in genetically defined cells of Escherichia coli. After treatment of cells with beta-lactams, we detected in the periplasm an increase of D-tripeptide (N-acetylglucosaminyl-1,6 anhydro N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid), D-tetrapeptide (N-acetylglucosaminyl-1, 6 anhydro N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid -D-alanine), and a yet-unknown anhydromuropeptide. We identified this anhydromuramylpeptide by fast atom bombardment-mass spectrometry as anhydromuramyl-pentapeptide. The amount of these molecules did not alter after treatment with cell wall-active non beta-lactams. The transmembrane protein AmpG transports not only D-tripeptide but also D-pentapeptide into the cell. In the cytoplasm these molecules are degraded into the corresponding monosaccharide peptides M-tripeptide (N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid) and M-pentapeptide (N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid-alanine-D-alanine). These findings indicate that besides M-tripeptide and D-tripeptide, probably M-tetrapeptide, D-tetrapeptide, M-pentapeptide, and D-pentapeptide are also signal muropeptides for beta-lactamase induction.