Metallothionein is a low-molecular weight protein involved in resistance to toxicity of heavy metals. Recent experimental evidences indicate that metallothionein appears to be a factor in determining the responsiveness of tumor cells to anticancer drugs, such as cisplatin and adriamycin. The resistance of tumor cells having high concentration of metallothionein to cisplatin has been observed in cultured cells and transplanted tumors. On the other hand, increase in metallothionein concentration in normal tissues of tumor-bearing animals prevents side effects of anticancer drugs. Metallothionein may be an important endogenous factor in cancer chemotherapy.