Various nitroimidazoles are used as antimicrobial and radiosensitizing agents in human medicine. Of these, 5-nitroimidazoles show high selective toxicity for anaerobic prokaryotes and eukaryotes. This review discusses the effects of 5-nitroimidazoles on microorganisms, i.e. microbicidal action, radiosensitizing action, inhibition of photosynthetic microorganisms, and induction of mutations. All these actions are enhanced by anaerobiosis and inhibited by aerobiosis or by the presence of certain reducible compounds. There is no indication that antimicrobial action could be dissociated from mutagenic properties. Relative resistance to 5-nitroimidazoles has been detected in some isolates of Bacteroides fragilis and Trichomonas vaginalis and was experimentally developed in B. fragilis and various trichomonads. Nitroimidazoles enter the microorganisms by diffusion. Facilitated transport has not been detected. The compound is reduced by low oxidation-reduction potential ferredoxin and similar electron transport components in the cell. In microorganisms highly susceptible to metronidazole, such compounds play a significant metabolic role. The reduction increases the outside-inside concentration gradient and thus drives further uptake. Certain short-lived products of the reduction are responsible for the cytotoxic action. In less susceptible organisms only limited amounts of such products are formed, and thus the cells are not killed but may undergo mutations. A major component of cytotoxicity is damage to DNA but other mechanisms cannot be excluded at present.