Protein C, one of the vitamin K-dependent plasma proteins synthesized in the liver, was measured immunologically in normal subjects (n = 20), patients with hepatocellular carcinoma (n = 60), liver cirrhosis (n = 60), acute hepatitis (n = 16), chronic hepatitis (n = 19), malignant neoplasms other than hepatocellular carcinoma (n = 35) and patients on warfarin treatment (n = 20). We also assayed gamma-carboxyglutamic acid-complete (carboxylated) protein C in these population by using a monoclonal antibody directed against human protein C, JTC-1, which recognizes the gamma-carboxyglutamic acid domain-related conformational change induced by metal ions. We demonstrated that the plasma of patients with hepatocellular carcinoma contains considerable amounts of gamma-carboxyglutamic acid-incomplete protein C, evidenced by the significantly reduced protein C:gamma-carboxyglutamic acid/protein C:antigen ratios in hepatocellular carcinoma as compared to those seen in normal controls, other liver diseases and other malignant neoplasms (p less than 0.01). In two patients with hepatocellular carcinoma with the reduced protein C:gamma-carboxyglutamic acid/protein C:antigen ratios, successful treatment (transcatheter hepatic arterial embolization or lipiodolization of antitumor agent) led to the very rapid normalization of the ratios. Intravenous administration of vitamin K, however, induced no such effects in three other patients with hepatocellular carcinoma with the abnormality. We conclude that the impaired vitamin K-dependent gamma-carboxylation observed in patients with hepatocellular carcinoma involves not only prothrombin, but also protein C, and that the impairment is not due to vitamin K deficiency.