[en] Some biological aspects of radiosensitization of microorganisms by NaCl at comparatively higher concentrations were studied. Among different strains of bacteria and yeasts, the radioresistant vegetative form was particularly sensitive to NaCl present during irradiations. With E. coli, such radiosensitizing effect of NaCl was abolished by mutations at the pol and rec loci or by the addition of certain solutes at the time of irradiation. The effective extracellular chloride concentrations for E. coli B/r were not less than 0.1 M. When actinomycin D and chloramphenicol were added during reincubation of E. coli B/r following irradiation in the presence of NaCl, the radiation lethal effect was enhanced, the extent of which was almost independent of doses of irradiation. From these results together with the previous data, the basic mechanism for NaCl radiosensitization was discussed with special references to a possible role of Cl₂^- radical anions as attacking species and to the inhibition of pol- and rec-dependent DNA repair systems as the primary cellular damage. (auth.)