The influence of the hyaluronan-binding protease (PHBSP), a plasma enzyme with FVII- and pro-urokinase-activating potency, on components of the contact phase (kallikrein/kinin) system was investigated. No activation or cleavage of the proenzymes involved in the contact phase system was observed. The pro-cofactor high molecular weight kininogen (HK), however, was cleaved in vitro by PHBSP in the absence of any charged surface, releasing the activated cofactor and the vasoactive nonapeptide bradykinin. Glycosoaminoglycans strongly enhanced the reaction. The cleavage was comparable to that of plasma kallikrein, but clearly different from that of coagulation factor FXIa. Upon extended incubation with PHBSP, the light chain was further processed, partially removing about 60 amino acid residues from the N-terminus of domain D5 of the light chain. These cleavage site(s) were distinct from plasma kallikrein or FXIa cleavage sites. PHBSP and, more interestingly, also plasma kallikrein could cleave low molecular weight kininogen in vitro, indicating that domains D5H and D6H are no prerequisite for kininogen cleavage. PHBSP was also able to release bradykinin from HK in plasma where the pro-cofactor circulates predominantly in complex with plasma kallikrein or FXI. In conclusion, PHBSP represents a novel kininogen-cleaving and bradykinin-releasing enzyme in plasma that shares significant catalytic similarities with plasma kallikrein. Since they are structurally unrelated in their heavy chains (propeptide), their similar in vivo catalytic activities might be directed at distinct sites where PHBSP could induce processes that are related to the kallikrein/kinin system.