SummaryKlebsiella and Enterobacter grow after exposure to large penicillin concentrations because of intrinsic resistance (resistance of the individual organism), penicillin-β-lactamase activity, or mutant overgrowth. In Klebsiella strains intrinsic resistances to benzylpenicillin and hetacillin were similar. In Enterobacter strains intrinsic resistances were distinctly lower to hetacillin than to benzylpenicillin and were reduced by a subinhibitory concentration of dicloxacillin. All strains had β-lactamase activity which was usually inhibited by dicloxacillin. Single step mutations to large increases in intrinsic resistance were common.