Bipyridinium compounds (viologens) have been shown to radiosensitize hypoxic Serratia marcescens cells by two components. These can be separated on the basis that only the one-electron reduced form of the compounds can penetrate the bacterium cell wall. One component is associated with sensitization at the membrane and the other with an internal site. The efficiency of sensitization at the membrane-associated site follows the order of increasing one-electron reduction potentials of the compounds. The one-electron reduced forms of the bipyridinium compounds are involved in a mechanism that reduces the initial level of sensitization. No additivity in sensitization is found on combining the bipyridinium compounds with other radiosensitizers, PNAP and Ro 07-0582 at concentrations of each, which will give sensitization to the level associated with the membrane site. It is concluded that all these electron-affinic compounds sensitize this site. The protective effect of added glycerol on sensitization by viologens is related to protection at the membrane-associated site.