Ciprofloxacin, a new quinolone derivative, was given prophylactically (500 mg twice daily) to 15 patients with acute leukemia during remission induction treatment. The effect on the microbial flora of the alimentary tract was evaluated. A rapid elimination ofEnterobacteriaceae was observed.Bacteriodes andClostridium species were not affected. Few ciprofloxacin resistant strains were isolated but did not lead to colonization. In a randomized study 56 patients with acute leukemia received either ciprofloxacin or trimethoprim-sulfamethoxazole plus colistin for prevention of infections. Six major infections occurred in 28 patients receiving ciprofloxacin, and 11 major infections in 28 patients receiving trimethoprim-sulfamethoxazole plus colistin. No infections caused by Gramn-egative bacilli were seen in the ciprofloxacin group compared to 17 in the other group (p <0.02). ciprofloxacin="" prevented="" colonization="" with="" resistant="" gram-negative="" bacilli="" while="" 12="" resistant="" colonizing="" strains="" were="" isolated="" from="" 10="" patients="" receiving="" trimethoprim-sulfamethoxazole=""><0.01). ciprofloxacin="" was="" better="" tolerated="" than="" trimethoprim-sulfamethoxazole+colistin;="" fewer="" side="" effects="">