Tissue thromboplastin apoprotein was partially purified from human brain. The apoprotein was recombined with mixed phospholipids to yield active thromboplastin. The recombined thromboplastin induced proteolytic activation of isolated human factor IX in the presence of factor VII and Ca2+. The clotting times of various deficient plasmas were determined as a function of apoprotein concentration, keeping the phospholipid concentration constant. The clotting times of a factor XII-deficient plasma were the same as those of a factor XII/factor IX-deficient plasma, except at very low apoprotein concentrations. However, under those conditions the difference in clotting times was independent of the presence of anti-factor VII serum. Similar observations were made for factor XI-deficient plasma in comparison with factor XI/factor IX-deficient plasma. These results indicate that activation of factor IX by factor VII/tissue thromboplastin does not significantly contribute to plasma coagulation.