Methylcobalamin is one of the coenzymatically active cobalamin derivates and required for the activity of the cytoplasmic enzyme methionine synthetase catalyzing the methylation of homocysteine into methionine. The effect of methylcobalamin on the proliferation of malignant cells has been examined. Methylcobalamin inhibited the proliferation of androgen-sensitive SC-3 cells (a cloned cell line from Shionogi mouse mammary tumor, SC115) in culture at the concentration of 100-300 micrograms/ml. An inhibitory activity of methylcobalamin on the proliferation was also observed in other cell lines (estrogen-sensitive B-1F cells from mouse Leydig cell tumor and MCF-7 cells from human mammary tumor) at the concentration of 500 micrograms/ml. Moreover, large doses of methylcobalamin injected intraperitoneally (100 mg/kg body weight/day) were non-toxic and suppressed the tumor growth of SC115 and B-1F cells in mice fed a vitamin B12 deficient diet. These results show that methylcobalamin inhibits the proliferation of malignant cells in culture and in vivo and propose the possibility of methylcobalamin as a candidate of potentially useful agents for the treatment for some malignant tumors.