The effect of cobalamin (vitamin B12) on the survival time of mice bearing P388 leukemia has been examined. Among the three cobalamins studied, the enzymatically active derivatives, methylcobalamin and 5'-deoxyadenosylcobalamin, were able to significantly increase the survival time of mice implanted intraperitoneally with the tumor cells. The pharmaceutical form, cyanocobalamin, was not active. The antitumor activity of these cobalamins may be associated with their functions in metabolism.