To determine whether nurses who worked in an intensive care unit (ICU) might acquire enteric colonization with Gram-negative aerobic bacteria from their patients, rectal swabs were obtained from the patients and nurses in the ICU, from nurses working in a coronary care unit (CCU) and from young women with urinary tract infection who had not been exposed to the hospital environment. The healthy women differed from the patients in that their enteric flora contained greater numbers of lactose fermenting bacteria, they were less often colonized by resistant organisms and when resistant bacteria were present they were less abundant than in the patients. Nurses who had been treated with antimicrobial agents during the preceding six months were colonized more often with antibiotic resistant enteric bacteria. There was no evidence of transmission of antibiotic resistant Escherichia coli from patients to nurses largely because this species had been virtually eliminated from the patients by intensive use of antimicrobial agents. Instead, several multiresistant species of Klebsiella, Citrobacter and Pseudomonas were isolated from the patients and ICU nurses. It was difficult, however, to define clearly patient-nurse controls who had no exposure to the ICU patients. These observations and those of other investigators emphasize the need for examination of control populations and of therapeutic use of antibiotics when examining the issue of environmental acquisition of antibiotic resistant bacteria.