Eleven patients undergoing cancer chemotherapy in isolator units were treated with an oral, nonabsorbable prophylactic regimen in an attempt to reduce the incidence of infections caused by the patient's own endogenous flora. Ninety-two percent of 98 strains of aerobic bacteria in the stool were completely suppressed, as well as 100% of the anaerobes and 54% of the fungi. Only 66% of the strains from throat cultures and 88% of the strains isolated from nose cultures were completely suppressed. The antibiotic regimen had to be continued during the entire study, although the patients remained in a sterile environment. The patients tolerated the antibiotics quite well, and severe toxicity was observed in only one patient after prolonged administration.