Selective antimicrobial decontamination with norfloxacin was compared with vancomycin/polymyxin for prophylaxis of bacterial infections in granulocytopenic patients. In the group of patients receiving norfloxacin, there were a lower number of acquired gram-negative bacillary organisms per patient (0.88 versus 1.86, p = 0.002), fewer patients with documented infection (16 of 36 versus 20 of 30, p = 0.12), and fewer cases of gram-negative septicemia (0 of 36 versus five of 30, p = 0.02). Norfloxacin was better tolerated (30 of 36 versus 16 of 30 patients highly compliant, p = 0.02), and associated with fewer gastrointestinal side effects (eight of 36 versus 14 of 36 patients, p = 0.07). These results suggest that norfloxacin is a more tolerable and efficacious oral antimicrobial agent than vancomycin/polymyxin for the prevention of serious gram-negative bacillary infections in granulocytopenic patients.