Plasma levels of prothrombin immunoreactive protein (factor II antigen) (II-Ag) and coagulant activity (II-CA) were determined in eight patients with acute hepatitis and in 29 patients with chronic liver disease (cirrhosis). The II-CA was reduced in 23 (62%), II-Ag in 17 (46%), and both were reduced in 13 (36%) of the cases. A disproportionate reduction was noted in 21 (57%); ie, there was more II-Ag found in comparison to the corresponding level of II-Ca. Ninety-six percent (23) of 24 patients with moderate to severe hepatocellular disease showed reduced II-CA levels; 63% (15) showed reduced II-Ag levels, with a disproportionate reduction in II-CA in 67% (16). These data suggest that reduced synthesis as well as impaired carboxylation of prothrombin precursor protein are factors contributing to the coagulopathy in patients with moderate to severe liver disease and that measurement of circulating levels of II-Ag may provide an excellent indication of hepatic synthetic capacity.