Besides a thermolabile inhibitor of tissue thromboplastin a thermostable inhibitor was found in human blood serum; molecular weight of the inhibitor (estimated by gel filtration) is 210000-235000 daltons, while molecular weight of a thermolabile inhibitor--480000 daltons. Inhibition of the thrombin formation system by the termostable inhibitor was reversible and was characterized kinetically as a mixed type of inhibition. Both inhibitors were found in blood of rats. Their content varied irrespectively of the state of hemocoagulation. Content of the thermostable inhibitor was distinctly decreased in blood serum after the blockade of reticuloendothelial system by administration of tripane blue.