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Blood biocompatibility of medical devices is in many ways dependent on surface characteristics and biochemical blood material interactions. In this study, the contact system, in which the activation of factor XII and plasma kallikrein is included, is highlighted. This article describes a simple chromogenic assay to determine the Hageman Factor fragment (HFf, or factor XIIf) and kallikrein activity invitro. The assay is based on conversion of Z-Lys-Phe-Arg-pNA.2HCl to which human factor XIIf and kallikrein appeared to have a high affinity. To discriminate between the serine proteases factor XIIf and kallikrein to cleave this substrate, aprotinin was added to one of two complementary samples. In this in vitro study, standardized disks from glass, high-density polyethylene (HDPE), polytetrafluoro ethylene (PTFE), and polydimethyl siloxane (PDMS) were studied for their capacity to generate factor XIIf and kallikrein in plasma. Kaolin was used as positive control. On glass disks the highest and on HDPE the lowest generation of factor XIIf and kallikrein were found, both with a ratio of 1:1. On PDMS and on PTFE disks protease activities were intermediate, but with a factor XIIf and kallikrein activity ratio of 1:2 and 1:4, respectively. Apparently because of the hydrophobic surface character of PDMS and PTFE, these surfaces absorb or fail to produce the factor XIIf. This assay appeared to be discriminative even for materials that are considered mild activators of the contact system and can therefore be used asa standard method to qualify biomaterials. NOTE: Factor XIIf (mel wt. 28,000) is designated in the literature as HFf, beta-factor XIIa, or factor XII(LMW) and factor XIIa (mel wt. 80,000) as HFa; factor XIIa(HMW) or alpha-factor XIIa. To avoid the unfashionable use of beta-factor XIIa or alpha-factor XIIa, we chose in this study the use of factor XIIf and factor XIIa, according to the article by R. A. Pixley et al., Blood, 66, 198-300 (1985). (C) 1994 John Wiley and Sons, Inc.