Effects of new anticancer drug Cis-platinum dichloride (CPDD) on yolk sac tumor were studied. Human yolk sac tumor serially transplanted in BALB/c nude mice (YST-1) was used as material in this experiment. Two administration methods of CPDD were examined and compared. 1) CPDD 5 mg/kg X 1 day X 4 2) CPDD i mg/kg X 5 days X 4. Tumor size was measured and serum AFP of nude mice was determined by radioimmunoassay method during experiments. At the end of experiments, treated tumor was resected and examined by light and electron microscopy. Results as follows: 1. CPDD was active in regressing yolk sac tumor at doses of 5 mg/kg given as a single treatment and doses of 1 mg/kg given daily for five days. It appeared that CPDD showed higher activity against tumors by daily injections at doses of 1 mg/kg. 2. Close correlation was recognized between tumor size and serum AFP of nude mice ((r = 0.93, p less than 0.001). Serum AFP of tumor-bearing nude mice revealed to be a useful marker of yolk sac tumor also in nude mice as in human patient. 3. Effect of CPDD on tumor cells was confirmed by light and electron microscopy.