The objective of this research program is to elucidate the mechanism by which vitamin K controls prothrombin synthesis in cultured mammalian liver cells. Abnormal prothrombins are synthesized in certain strains of rats and in some humans in response to warfarin and will be looked for in fetal and neoplastic tissue. They will be studied to determine the extent to which the vitamin K-warfarin control system for prothrombin synthesis is altered or modified. Prothrombin from both rat and human plasma has been isolated and yields antibodies in rabbits. Prothrombin in nanogram quantities can be detected in biological fluids, on ribosomes and in cells by radioimmunoassay and immunofluorescence. Observation of prothrombin biosynthesis in the presence of classical inhibitors of protein synthesis should aid in revealing the nature of the control site for vitamin K. Normal rat and human prothrombins will be characterized by their proteolytic fragments, amino acid composition, peptide mapping and localization of antigenic determinants. This will provide the basis for classifying abnormal prothrombins arising from fetal, neoplastic, or warfarin treated cell suspensions. It is possible that a form of vitamin K altered regulation may account for the appearance of these abnormal prothrombins.