gamma-Carboxyglutamic acid residues on prothrombin are synthesized from glutamic acid on a prothrombin precursor in the liver through a vitamin K-dependent carboxylase. In the absence of vitamin K or in the presence of vitamin K antagonists, an inert form of prothrombin - abnormal prothrombin - circulates in the blood. We have developed specific immunoassays for native and abnormal human prothrombin. The prothrombin concentration in our normal subjects was 108 +/- 19 microgram per milliliter. The abnormal-prothrombin concentration varied over four orders of magnitude between the limits of detection in normal plasma and the level in patients with cirrhosis (0 to 5 microgram per milliliter), acute hepatitis (0 to 33 microgram per milliliter), or vitamin K deficiency (32 to 100 microgram per milliliter) and in those treated with sodium warfarin (12 to 65 microgram per milliliter). These studies indicate that abnormal prothrombin is not a component of normal plasma but appears in a variety of hepatic and nutritional disorders characterized by impaired hepatic vitamin-K-dependent carboxylation.