The supernatant obtained by centrifugation of a rat liver homogenate at 100000g for 1h contained a heat-labile macromolecular inhibitor of the thrombin-fibrinogen reaction. 2. The inhibitor was purified to electrophoretic homogeneity by repeated preparative polyacrylamide disc electrophoresis. Inhibition was observed with purified inhibitor equivalent to about 1mug of protein/ml. 3. The inhibitor had a pI of 3.50-3.75, a molecular weight (from sodium dodecyl sulphate-polyacrylamide-gel electrophoresis) of 72000+/-3000 and was inactivated by p-hydroxymercuribenzoate or 5,5'-dithiobis-(2-nitrobenzoic acid). 4. Kinetic studies revealed a non-competitive inhibition, with the inhibitor probably acting on the thrombin-fibrinogen complex.
