The antimicrobial susceptibilities of 368 E. coli strains isolated from well neonates, well pediatric outpatients, and ill neonates were determined, and the incidence of antibiotic-resistant strains was correlated with epidemiologic data. Ninety-five per cent of these isolates were susceptible to kanamycin and 98 per cent to gentamicin. Most of the difference in susceptibility of these E. coli strains to kanamycin and gentamicin appeared to be on the basis of weight for weight activity. Kanamycin-susceptible strains were isolated from 96 per cent of well neonates and well outpatients, and 94 per cent of ill neonates, age three days or less. An increase in the incidence of resistance to kanamycin, streptomycin, and ampicillin among strains isolated from neonates was related to prolonged hospitalization and/or prior antibiotic therapy. This study demonstrates the need for analyzing antimicrobial susceptibility data from newborn nurseries in an epidemiologic context.