Vitamin K-1 epoxide, the major metabolite of vitamin k-1, has similar activity to vitamin K-1 in inducing prothrombin synthesis and protein carboxylation. The high activity of K-1 epoxide could be due to its conversion to vitamin K-1 hydroquinone without going through vitamin K-1. A logical intermediate in this conversion would be vitamin K-1 epoxide-1,4-diol. The epoxide diol was synthesized and clearly stimulated prothrombin synthesis in vitamin K deficient rats at a minimum dose of 100 micrograms/kg body weight. Since vitamin K-1 produced a similar response at a minimum dose of 1 microgram/kg, the epoxide diol had about 1% of the activity of vitamin K-1. [3H]K-1 or [3H]epoxide could not be detected as metabolites of [3H]epoxide diol indicating that the activity of epoxide diol was probably not due to its conversion to K-1 hydroquinone, since any [3H]hydroquinone formed would be oxidized in air to [3H]K-1 during analysis. Vitamin K-1 epoxide diol represents a new type of structure possessing vitamin K activity. It is probably not active itself but has to be converted to an active compound since there is a delay in the response to the diol and the activity is completely blocked by Warfarin.