In a pilot study, a prophylactic regimen including ciprofloxacin and amphotericin B was applied in 102 consecutive patients on artificial respiration for greater than or equal to 5 days to prevent respiratory tract infection with aerobic Gram-negative bacilli. Ciprofloxacin was given twice a day, as 500 mg through a gastric tube or 200 mg intravenously, and both applications led to negative cultures for aerobic Gram-negative bacilli from faeces and throat, except for a few periods of carriage lasting only a few days. No patient acquired respiratory tract infection with one of the Enterobacteriaceae or Pseudomonadaceae after 4 days of artificial respiration. In contrast to other prophylactic regimens in intensive care patients, this regimen is relatively simple and effective. These preliminary data suggest that this regimen should be studied further with special emphasis on the induction of resistance in Intensive Care Units using prospective, double-blind study designs.