The toxic actions of the "nitro" radiosensitizers, metronidazole and misonidazole on the bacteria E. coli B/r and Serratia marcescens have been investigated under anareobic and aerobic conditions. The rates of reduction of the drugs by suspensions of these bacteria as well as by suspensions microorganisms from the rat caecum have been measured. Both drugs were reduced or were toxic only under anaerobic conditions. In all instances misonidazole was reduced more rapidly than metronidazole but metronidazole was more toxic. It is suggested that these phenomena may model those occurring with hypoxic mammalian cells in vivo and that care should be taken before automatically extrapolating in vitro data to the in vivo situation.