Over the past few years the introduction of chromogenic substrates has facilitated studies on components of the plasma defence systems. During this period it has become apparent that there is a need for simple, easy to perform, assays for measuring components of these systems. We have therefore developed simple assays for determining prekallikrein (PKK), kallikrein inhibition (KKI) and kailikrein-1ike activity (KK). These assays have a plasma dilution step, followed by repeat single volume

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... tings of reagents. Both "rate" and "end-point" assays have been developed. For the PKK assay a new activator has been developed which contains Hageman factor and high mol. wt. kininogen. This overcomes the problems found by several groups with other activators when studying patients with low plasma levels of Hageman factor, PKK and high mol. wt. kininogen. For the KKI assay a stable preparation of human plasma KK has been produced. This assay detects mainly Cl'- esterase inhibitor. The assay for KK-like activity is required where activation of PKK has occured and spontaneous activity towards the substrate is found. Plasma samples from 25 normal donors were tested for PKK, KKI and KK. The PKK and KKI values were calculated from standard curves made with a standard plasma. When compared with PKK and C1INH antigen levels correlations of 0.84 and 0.85 were obtained. All of the plasmas had KK activities lower than 5% of the total PKK levels.These assays should prove valuable in the rapid evaluation of the plasma KK system in patients.