Most of Enterobacter cloacae strains produce chromosomally determined class I beta-lactamases when they are exposed to beta-lactams. Imipenem is a strong inducer of these enzymes but is poorly affected by them. We compared the effect of imipenem on inducible, non-inducible and stably derepressed strains of E. cloacae using the killing curve system. With antibiotic concentrations of 0.5 mg/l or more, an intense dose-dependent bactericidal effect was observed within 4 to 6 hours. However the bactericidal activity was incomplete. With an inoculum as low as 10(5) bacteria/ml a regrowth was observed after 24 hours. Among the surviving bacteria imipenem had the same bactericidal kinetic than on the parental strain. Similar results were observed with inducible, non-inducible and stably derepressed strains.