The effect of Ehrlich ascites tumor growth on selenium-turnover rates and selenium-75 distribution in liver, kidney, and immunological tissues (spleen, thymus, and lymph nodes) was investigated in Swiss Webster mice that had been prelabeled with selenium-75. Ehrlich ascites tumor caused a decrease in the selenium-75 content of liver, kidney, and thymus; it also decreased the rate of the total-body selenium-turnover. In liver, depletion of selenium-75 was almost as great as that produced by a selenium and vitamin E-deficient diet. When mice had been fed an antioxidant-deficient diet, considerable quantities of selenium-75 were accumulated by the tumor; the specific activity of the tumor increased 9-fold over that in antioxidant-supplemented mice. The same diet produced a premature, and in some cases drastic, contraction in tumor volume. The possible significance of tumor-induced antioxidant deficiencies to the etiology of certain paraneoplastic syndromes is discussed.