SUMMARYThe thermal resistance at 100°C ofClostridiun sporogenesP.A. 3679 was determined after the spores had been irradiated with gamma rays or cathode rays under various environmental conditions.The pre‐irradiation treatment sensitized the spores to heat, and spores irradiated either in air or under vacuum were heat‐sensitized to the same extent. The heat sensitization induced was slightly greater when the spores were irradiated at a pH near neutrality, than at an acid pH. Spores suspended in phosphate buffer (pH 7.0) were heat‐sensitized to a greater degree than spores suspended in nutrient broth or pureed ham. The thermal resistance of the irradiated spores was not influenced by the concentration of viable spores in the suspension (phosphate buffer), but as the proportion of dead cells and dead spores increased with increasing doses of irradiation, the thermal resistance also increased.Spores irradiated with 660,000 rep at 66–68°C were more heat‐sensitive than spores irradiated at room temperature or in the frozen state. Below this irradiation level this effect was
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