Dose-survival curves of four strains of E. coli-B/r <tex-math>${\rm hcr}^{-}\ {\rm try}^{-},\ {\rm B}_{{\rm s}-1}$</tex-math>, B/r CSH, and <tex-math>$15{\rm T}^{-}$</tex-math>-were obtained in the presence and in the absence of nontoxic levels of several thiol-binding agents-N-ethylmaleimide (NEM), iodoacetamide (IA), and hydroxymercuribenzoate (HMB). The degree of radiosensitization by these agents was estimated from the increase in slope of the dose-survival curve, under anoxic and aerobic conditions. Chemical sensitization of both radioresistant and radiosensitive strains of bacteria with the thiol-binding enzyme poisons NEM, IA, and HMB has been observed. NEM sensitized only under anoxic conditions, IA sensitized under both anoxic and aerobic conditions, although to a much greater extent aerobically, and HMB sensitized only under aerobic conditions. The formation of long-lived radiolytic products toxic to bacteria is observed in sensitization by IA. When sensitization occurs, the magnitude of the dose-modifying factor is equal to or greater than the oxygen enhancement ratio. Inhibition of an energy-requiring enzymic repair process does not seem to be involved as a primary mechanism of sensitization. Interference by thiol-binding agents with a "rapid repair" process which is different from the usual "enzymic repair," and which operates in radiosensitive as well as in radioresistant bacteria, is suggested.

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