The efficacy of oral prophylaxis with ciprofloxacin, ofloxacin or pefloxacin was assessed in preventing bacterial infection in neutropenic patients with treatment being allocated randomly before beginning chemotherapy. Bacteraemia developed in six of 78 episodes (8%) treated with ciprofloxacin, in eight of 80 (10%) allocated to ofloxacin and in 12 of 77 (16%) when pefloxacin was given. However, there were no episodes involving Gram-negative bacilli among those given ciprofloxacin whereas three and seven episodes occurred in patients given ofloxacin or pefloxacin respectively (P = 0.013). With the exception of Pseudomonas aeruginosa, all potential pathogens isolated were resistant to all three fluoroquinolones. Faecal anaerobes were not affected by treatment with pefloxacin whereas their total numbers were reduced in 12 cases who had received ofloxacin and in nine cases who had been given ciprofloxacin (P = 0.002). Fourteen patients (18%) were colonized with pefloxacin resistant P. aeruginosa at the end of treatment with this agent compared with only two and five of those given ciprofloxacin or ofloxacin respectively. A similar trend was seen with other resistant Gram-negative bacilli colonizing 14%, 20% and 23% of patients for ciprofloxacin, ofloxacin and pefloxacin, respectively. Ciprofloxacin was therefore superior to the other two fluoroquinolones in preventing infections due to Gram-negative bacteria in this population of neutropenic patients.