SUMMARY Salmonella typhimurium was irradiated in phosphate buffer, pH 7.6, containing various levels of vitamin Ks over a temperature range of 32 to 120°F either in air or under vacuum. Vitamin K5 exerted a radiosensitizing effect under vacuum and a radioprotective effect in air. The radiosensitizing effect decreased with increasing temperature. Vitamin KS had no radiosensitizing activity, or possibly a slight protective effect, in whole egg irradiated either in air or under vacuum.