Induction of the cloned ampC Citrobacter freundii β-lactamase in Escherichia coli by 6-aminopenicillanic acid was prevented by periplasmic TEM β-lactamase in the same cell. In contrast, cytoplasmic TEM β-lactamase did not prevent induction. This result implies that entry of the β-lactam into the cytoplasm is not necessary for induction of ampC.