Patients admitted to a medical intensive care unit (29), a surgical intensive care unit (29) and a general medical ward (34) were studied to determine the frequency of colonization by Staphylococcus aureus, Staph. epidermidis and enterococci. Cultures of anterior nares, pharynx, axilla, periurethral tissues, and rectum were done within 24 h of admission and every 48–72 h thereafter for the duration of hospitalization. We found Staph. aureus colonization of pharynx, axilla, periurethral tissues and rectum to be equally frequent in patients with and without nasal colonization. Staphylococcus epidermidis was commonly recovered from all sites except the pharynx. The 49 strains of this organism acquired in the hospital were significantly more often resistant to nafcillin, cephalothin, chloramphenicol, trimethoprim-sulfamethoxazole and gentamicin than were 199 isolates recovered from patients at the time of admission. Enterococci were commonly isolated from rectal swabs and, less often, from other sites. Acquisition of enterococcal colonization during hospitalization was correlated significantly with antibiotic administration.