Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblot analysis of plasma thrombomodulin concentrate revealed that four degraded forms of thrombomodulin with different molecular weights are present in plasma. Plasma concentrations of thrombomodulin in patients with various diseases were measured by two methods of enzyme- linked immunosorbent assay using monoclonal antibodies. One method measures intact thrombomodulin and degraded forms of thrombomodulin; the other does not detect the two smaller degraded forms of thrombomodulin present in plasma. The results indicated that thrombomodulin was increased in the circulating blood of patients with disseminated intravascular coagulation syndrome, pulmonary thromboembolism, adult respiratory distress syndrome, chronic renal failure, or acute hepatic failure. The different values obtained by the two methods indicate that the increase of plasma thrombomodulin found in these patients was mainly due to an increase of the smaller fragments of degraded forms, suggesting that the release of thrombomodulin from endothelial cells was accelerated in various disease states by proteolytic activity generated on the surface of the endothelium and may be removed from the circulation mostly by the kidneys and liver.