Injury in Escherichia coli irradiated by far-infrared radiation (FIR) was examined. We used penicillin G (PCG), chloramphenicol (CP), rifampicin (REP) and nalidixic acid (NA) as selective reagents, and tried to evaluate the injury in bacteria irradiated by FIR on the basis of the changes in the sensitivity to selective reagents. The validity of this method was examined using UV pasteurization. E. coli irradiated by UV became more sensitive to CP and NA, which indicated that UV irradiation damaged nucleic acid in E. coli. This result agrees with those demonstrated by many researchers and suggests this method is applicable to useful evaluation of the damaged part in bacteria induced by stresses. E. coli irradiated by FIR became more sensitive to RFP and CP, which suggests that FIR irradiation damages RNA polymerase and ribosome in E. coli. Though similar changes in the sensitivity were obtained in the case of thermal conductive heating, the pasteurization effect of RR irradiation was much greater than that of conductive heating.