Vitamin A (alcohol) pretreatment of L1210 leukemia cells in vivo enhanced the antitumor effects of X-irradiation. This effect of the vitamin was dose and time dependent. For a given dose of X-rays, maximum inhibition of reproductive capacity and survival of the leukemic cells were observed in culture pretreated 4 hours with the vitamin. In combined X-ray and vitamin A treatments, maximal inhibition of reproductive capacity and survival, as compared with treatment with X-rays alone, occurred with a 4-hour vitamin A pretreatment. As this period of 4 hours coincided with the peak of lysosomal enzyme release induced by the vitamin, a function for acid hydrolases in tumor-cell killing by X-rays was postulated. From these results, however, it was not clear whether the effect of vitamin A in tumor-cell killing was additive or whether a true sensitization to radiation also took place.