A mutant strain of Escherichia coli with reduced susceptibility to imipenem, designated TL2740, was selected following serial passage of the parent strain, E. coli C600, in broth containing increasing concentrations of the carbapenem; the MIC of imipenem for TL2740 was eight-fold greater than that of the parent strain. The mutant also exhibited reduced susceptibilities to panipenem and biapenem and high-level resistance to mecillinam, but was as susceptible to meropenem, ceftazidime, piperacillin and the other beta-lactams tested as strain C600. The affinity of penicillin-binding protein (PBP) 2 of TL2740 for imipenem and meropenem was ten-fold less than that of C600, thereby providing an explanation for the mutant's reduced susceptibility to some carbapenems and mecillinam. However, this theory was confounded by the observation that the in-vitro activities of meropenem against both parent and mutant strains were virtually the same and by the fact that PBP 2 is the principal target of the antibiotic. Imipenem and aztreonam, which bind to PBP 2 and PBP 3 respectively, demonstrated synergic activity when tested in combination against both C600 and TL2740. These results suggest that the potent activity of meropenem against the mutant strain might also be due to a synergic effect resulting from simultaneous binding to both PBP 2 and PBP 3 and that the variable activities of the carbapenems against TL2740 were related to their different PBP binding profiles. Compared with C600, TL2740 appeared shorter on electron microscopy and had a longer generation time, discrepancies which are compatible with defective PBP 2 activities in the mutant strain. We also identified three clinical isolates of E. coli with beta-lactam susceptibility profiles which resembled that of TL2740 i.e. high-level resistance to mecillinam and low-level resistance to carbapenems, with the exception of meropenem to which these strains were susceptible; in common with TL2740, the combination of imipenem and aztreonam was synergic against these isolates. The genetic basis of resistance in all of the mecillinam-resistant strains, including TL2740, mapped close to lip at 15' on the E. coli chromosome with transductional analysis. The results strongly suggest that the reduced susceptibilities of the clinical isolates to carbapenems were due to mutations in the genes encoding the PBP 2s of these strains which affected their affinities for beta-lactam antibiotics.