The effect of intraperitoneal and oral pretreatment with combined vitamins C and K3 on the single dose radiotherapy of a transplantable solid mouse tumor have been investigated. Groups of mice bearing intramuscularly transplanted liver tumors, were orally and parenterally pretreated with combined vitamins C and K3 and locally irradiated with single doses of 20, 30, or 40 Gy of X-rays. After this treatment tumor dimensions were measured twice weekly and the approximate tumor volume in groups of pretreated vitamins and irradiated mice was compared to the groups of mice only irradiated and to the absolute control groups without any therapy. This nontoxic pretreatment produced statistically significant potentiation of radiotherapy induced by 20 to 40 Gy of X-rays doses in groups of 11 to 20 mice. Combined vitamins C with K3 most probably constitute a redox-cycling system producing hydrogen peroxide and other active oxygen species to which cancer cells are selectively sensitive due to their frequent deficiency in enzymatic defense system against free oxyradicals agression. A possible introduction of such nontoxic and selective potentiation procedure into classical protocols of human cancer therapy appears to be generally accessible and without any additional risk for patients.