Factor VII clotting activity increases .apprx. 5-fold when blood is clotted in glass. Prior studies suggested that this results from activation induced by activated factor IX (IXa). However, in purified systems containing phospholipid and Ca, activated factor X (Xa) activates factor VII rapidly. Activation of factor VII by IXa and Xa was studied in systems using purified human factors. Concentrations of IXa and Xa were calculated from total activated protein concentrations rather than from active site concentrations. In the presence of phospholipid and Ca, both IXa and Xa activated factor VII 25-fold; Xa was roughly 800 times more efficient than IXa. Without added phospholipid, activation of factor VII by Xa and IXa was markedly slowed, and Xa was roughly 20 times more efficient than IXa. When phospholipid and Ca were omitted, activation of factor VII by either enzyme was negligible. Adding normal prothrombin, but not decarboxylated prothrombin, substantially slowed activation of factor VII by both Xa and IXa. Adding thrombin-activated factor VIII and antithrombin-III did not change rates of factor VII activation by either enzyme. These results from purified systems do not provide an explanation for the prior data from plasma systems.