Enterobacter cloacae strains dominated the aerobic faecal flora of 8.3% of 953 infants discharged from 32 Swedish neonatal intensive care units and the susceptibility of these strains to seven beta-lactam antibiotics was determined. Isolates from infants treated with cefuroxime showed slightly increased MICs only to ampicillin, cephalexin and cephalothin as compared to isolates from untreated infants matched for ward and time of sampling (P = 0.02). In contrast, E. cloacae isolates from ampicillin treated infants showed markedly elevated MICs of all agents tested including piperacillin, cefuroxime, cefotaxime and ceftazidime as compared to those from control neonates (P values between 0.001 for ampicillin and 0.017 for cefotaxime). Thus, E. cloacae with cefotaxime MICs as high as 512 mg/L were isolated only after ampicillin therapy. The resistant strains were negative in a colony DNA hybridization assay using gene probes for the plasmid beta-lactamases TEM-1, OXA-1 and SHV-1. The resistant strains also showed only one beta-lactamase band when crude cell sonicates were analysed by isoelectric focusing, and were not found in other infants in the same ward. The results indicate that the selection of chromosomal E. cloacae mutants, presumably with stably derepressed beta-lactamase production, in the faecal flora of neonates is rare during treatment with cefuroxime and more common during ampicillin therapy.