Ampicillin at a 50-mug/ml concentration effects a doubling of the autolysis rate of cell envelopes isolated from Klebsiella pneumoniae Mir A12. This antibiotic increases also by two- to threefold the depolymerizing activity of both K. pneumoniae and Escherichia coli crude extracts on a labeled peptidoglycan-lipoprotein complex. Several other penicillins also activate autolysins.