During the early events of coagulation of human blood by the intrinsic pathway, factor XII is activated to a form which can activate factor XI, and is proteolytically fragmented to smaller species (30,000 daltons and 70,000 daltons) which have lost most of the ability to activate factor XI but which can activate prekallikrein rapidly. The effect of these fragments on factor VII was studied. It was found that these Hageman factor fragments promoted rapid proteolysis of one-chain factor VII to a more active two-chain form. The amino-terminal sequences of the chains of activated factor VII were found to be Ala- Asx-Gly- and Ile-Val-Gly-, the same as were earlier observed after activation of factor VII by activated factor X. This finding indicates that initiation of coagulation by the intrinsic pathway also primes the extrinsic pathway.