Twenty-four neutropenic patients receiving intensive chemotherapy for acute non-lymphocytic leukemia were studied in a randomized trial comparing ciprofloxacin with polymyxin B for prevention of infections. Both groups (12 patients each group) received amphotericin B for antifungal prophylaxis. 20 febrile episodes occurred in 22 courses of oral prophylactic ciprofloxacin and 22 occurred in 24 courses of oral prophylactic polymyxin B. Patients receiving ciprofloxacin had a mean time to the first infection-related febrile episode of 7.2 days, compared with 4.3 days for the polymyxin B group (p less than 0.01). Patients receiving ciprofloxacin also had fewer days of fever (average 6.5 days versus 9.8 days for the polymyxin B group, p less than 0.02). Duration of administration of parental antibiotics were also shorter in the ciprofloxacin group (p less than 0.001). Although modifications of the empiric antibiotic regimen were required more frequent in patients receiving polymyxin B, this did not reach statistical significance. These results suggest that ciprofloxacin is a more efficacious oral antimicrobial agent than polymyxin B for the prevention of infections in neutropenic patients with acute non-lymphocytic leukemia.