24R,25(OH)2D3, one of the endogenous active metabolites of vitamin D3, showed suppressive effects on the proliferation of various tumor cells in vitro and a prolonging effect on the survival of P-388 bearing mice in vivo. Lewis lung carcinoma was found to cause hypercalcemia in tumor bearing mice. K-DR (24R,25(OH)2D3 (prepared by Kureha Chemical Ind.) significantly prolonged the survival of mice with Lewis lung carcinoma. K-DR also showed a suppressive effect on the growth of human osteosarcoma transplanted in nude mice.