Norfloxacin (NOR) or cotrimoxazole (TMP/SMX) were randomly administered to 59 granulocytopenic patients with acute leukemia for prevention of bacterial infections. Nineteen NOR patients (65%) and 22 TMP/SMX patients (73%) complained of febrile or infectious episodes during the study. The mean incidence of febrile complications per patient was higher in the TMP/SMX group: 1.05 vs 0.68 (p less than 0.05). Eleven of 16 microbiologically documented infections in the TMP/SMX group and 7 of 11 in the NOR group were caused by gram negative bacilli (GNB). NOR recipients had fewer days of fever, fewer days on parenteral antibiotics and a lower proportion of time spent febrile. Fecal surveillance cultures showed intestinal GNB colonization in 42/80 specimens in the TMP/SMX group (resistant strains: 93%) and in 8/75 specimens in the NOR group (1 resistant strain). Overall, NOR seems to be effective in eradicating GNB from the digestive tract without selection of resistant strains and in preventing febrile episodes in neutropenic patients.