The synthesis of factor VIII and other coagulation factors has been studied using an isolated, perfused rat liver. Synthetic function of the liver was validated by adding [35S]L-methionine to the perfusion medium and performing two-dimensional crossed immunoelectrophoresis and autoradiography on samples obtained during perfusion. Progressive incorporation of radioactivity into plasma proteins was demonstrated. This was inhibited by cycloheximide. Coagulation factor assays demonstrated synthesis of factors II, IX and X and of factor V and procoagulant factor VIII (VIIIC). Synthesis of factor VIII related antigen (VIIIRAg), measured in an immunoradiometric assay, was not significantly demonstrated. Addition of warfarin to the perfusion medium inhibited the synthesis of factors II, IX and X but not of factors V and VIII. Cycloheximide completely inhibited synthesis of all coagulation factors but actinomycin acted only after a latent period. Reticuloendothelial cell blockade was attempted by adding ethionine to the perfusion medium or by administration of Indian ink to the donor animals prior to removal of the livers. In these instances synthesis of factor V and factor VIIIC was inhibited but not that of factors II, IX and X. The results confirmed the functional capacity of the isolated liver for synthesizing proteins and the vitamin K dependent coagulation factors, and suggested similar kinetic features for the synthesis of factors V and VIIIC. Failure to detect significant VIIIRAg synthesis in these experiments is consistent with the hypothesis that this protein is released by vascular endothelial cells throughout the body and is activated or joined to VIIIC or stimulates its production in the liver.