The relation between local antibiotic consumption and faecal carriage of beta-lactam-resistant strains of Escherichia coli and Klebsiella spp was studied in 953 children discharged from twenty-two neonatal units. An increased rate of carriers of Klebsiella spp strains resistant to ampicillin, cefuroxime, and cephalexin and of E coli strains resistant to ampicillin was associated with the use of ampicillin, with or without gentamicin. Ampicillin and cephalosporin (86% cefuroxime) consumptions were inversely related, which probably explained the paradox that cephalosporin use (and therefore less ampicillin use) was negatively correlated with the carriage of Klebsiella spp strains resistant to ampicillin, cefuroxime, and cephalexin. The ecological impact of antibiotics was much the same among treated and untreated babies in each unit. The findings show a greater risk of drug resistance due to a related agent rather than to the drug itself and a clear indirect impact of an antibotic on the microflora of untreated patients. Ampicillin-based regimens are more likely than cefuroxime to produce drug-resistant strains in the newborn.