Resistance to imipenem and meropenem, reported sporadically in Enterobacteriaceae and more frequently in Pseudomonas aeruginosa, can be caused, among other mechanisms, by the combination of changes in permeability and hyperproduction of inducible chromosomal beta-lactamases. In this study, the in vitro activity of imipenem and meropenem was analysed by the agar dilution method against cefotaxime, ceftazidime, and aztreonam resistant clinical strains of Enterobacteriaceae (n = 202) and P. aeruginosa (n = 90). This phenotype is consistent with the hyperproduction of group 1 chromosomal beta-lactamases and was previously determined in stably derepressed mutants in the same species, obtained from strains with inducible beta-lactamase expression by selection with cefotaxime and ceftazidime. Likewise, the activity of imipenem and meropenem against the same number of clinical isolates susceptible to cefotaxime, ceftazidime, and aztreonam was evaluated. In general, imipenem and meropenem showed an excellent activity, which was intrinsically greater for meropenem against Enterobacteriaceae and P. aeruginosa organisms. Nevertheless, imipenem and meropenem activity was slightly affected on cefotaxime, ceftazidime, and aztreonam resistant isolates of E. cloacae (MIC90, 1 and 0.2 microgram/ml, respectively), E. aerogenes (1 and 0.2 microgram/ml), C. freundii (1 and 0.1 microgram/ml), M. morganii (1 and 0.5 microgram/ml), and S. marcescens (4 and 0.5 micrograms/ml). On the other hand, the activity of imipenem and meropenem against ceftazidime and aztreonam resistant isolates of P. aeruginosa was more significantly affected, with MIC90 values of 64 and 16 micrograms/ml, respectively.