The effect of L-5-hydroxytryptophan (L-5-HTP) against the depression of tumor-resisting power in host animal, which is induced by a whole body x-irradiation of 500 R, was studied. Host animal and tumor cell used were LAF, hybrid mouse and mastocytoma cell, respectively. The i.p. injected dose of L-5-HTP was 6.25 mg per mouse, which is most effective for protecting mice from radiation-induced hematopoietic death. In the experimental conditions, the agent was not effective to prevent the radiation-induced depression of tumor-resisting power of the animal.