Blood coagulation factor XII (FXII, Hageman factor) is a plasma serine protease which is autoactivated following contact with negatively charged surfaces in a reaction involving plasma kallikrein and high-molecular-weight kininogen (contact phase activation). Active FXII has the ability to initiate blood clotting via the intrinsic pathway of coagulation and inflammatory reactions via the kallikrein-kinin system. Here we have determined FXII-mediated bradykinin formation and clotting in plasma. Western blotting analysis with specific antibodies against various parts of the contact factors revealed that limited activation of FXII is sufficient to promote plasma kallikrein activation, resulting in the conversion of high-molecular-weight kininogen and bradykinin generation. The presence of platelets significantly promoted FXII-initiated bradykinin formation. Similarly, in vitro clotting assays revealed that platelets critically promoted FXII-driven thrombin and fibrin formation. In summary, our data suggest that FXII-initiated protease cascades may proceed on platelet surfaces, with implications for inflammation and clotting.