During the last few years the impact on the human endogenous microflora by norfloxacin has been studied by several investigators. The use of norfloxacin for selective decontamination in compromised patients and for prophylaxis of recurrent urinary tract infections has also been investigated. This review article summarizes the published data from these studies. The results show that the oropharyngeal flora is only slightly affected by norfloxacin. In the intestinal flora, the Gram-negative aerobic microorganisms are strongly suppressed during administration of norfloxacin. The impact on the Gram-positive aerobic flora is minor, while the anaerobic intestinal microflora is virtually unaffected by the administration. In two studies where norfloxacin was given prophylactically to granulocytopenic patients, the development of aerobic Gram-negative infections was suppressed and overall morbidity decreased. Norfloxacin for long-term prophylaxis of infections in artificially ventilated patients in intensive care units was reported to give encouraging results in one investigation. Two studies indicated that long-term norfloxacin prophylaxis reduces the rate of recurrent urinary tract infections. Norfloxacin administration seems not to predispose for development of resistance to 4-quinolones nor to other commonly used antimicrobial agents.