This study sought to quantitatively describe the spectrum of constitutive and inducible beta-lactamase activity present in a tertiary care center's population of Enterobacter species. beta-Lactamase activity in the absence and presence of 2 recognized beta-lactamase-inducing antibiotics, cefoxitin and imipenem, was measured. The Enterobacter cloacae (n = 35) population was strikingly bimodal, expressing 'all-or-none' cross-resistance to beta-lactams (except imipenem) corresponding to the baseline level of beta-lactamase expression. E. aerogenes (n = 14) displayed a less dichotomous pattern of resistance, and MICs of beta-lactam antibiotics were less strongly related to the magnitude of enzyme activity. We conclude that our nosocomial population of E. cloacae, like strains rendered resistant to beta-lactams in vitro, is largely dependent upon beta-lactamase as a mechanism of this resistance. Furthermore, we document the presence of a large subpopulation of beta-lactam-susceptible isolates possessing inducible beta-lactamase and therefore subject to selection for enzyme derepression.