Vitamin K is a substrate for a liver microsomal enzyme that catalyzes the conversion of specific glutamyl residues to gamma-carboxyglutamyl residues in a limited number of proteins. These include the vitamin K-dependent clotting factors: prothrombin (factor II), factor VII, factor IX, and factor X. In the absence of vitamin K, nonfunctional clotting factors are synthesized and hemorrhage can result. A Recommended Dietary Allowance of 1 micrograms/kg body weight has been established for vitamin K. Advances in analytic techniques and more sensitive clotting factor assays will make it possible to define the human requirement for this vitamin more accurately. A limited amount of data on the vitamin K content of foods is now available and reasonable estimates of intake can be calculated. Green leafy vegetables constitute the major source of vitamin K in the diet.