Serum levels of F protein, a 44 kD cytoplasmic protein mainly found in hepatocytes, become elevated during episodes of graft dysfunction following orthotopic liver transplantation. In a study of 27 liver transplant recipients, the rise in F protein did not precede rises in the other conventional biochemical indices of hepatic dysfunction. Serum F protein concentration significantly correlated with serum levels of aspartate aminotransferase, gammaglutamyltransferase, alkaline phosphatase, and bilirubin (allP< 0.001) and also with the prothrombin time (P = 0.048). Despite its high concentration in liver cells, this marker does not provide any additional benefit in the diagnosis of graft dysfunction or in monitoring liver allograft function following transplantation.