Immune aggregates of bovine serum albumin (BSA) and rabbit anti-BSA presumably adsorb Hageman factor (factor XII) and induce its conversion t o prekallikrein activator (PKA), which can be eluted from the precipitates and purified by chromatography. N o clotting studies could be done with the material desorbed from the immune aggregates incubated with serum, because thrombin was present in t h e preparations.When Al(OH)3-adsorbed plasma (free of prothrombin) was incubated with celite, large quantities of PKA were recovered in the celite eluates, together with activated factor XI1 (XIIa), activated factor XI (XIa) and kallikrein. These components could be separated by anion exchange chromatography and gel filtration. PKA had a molecular weight of approximately 31 000, 33 000 and 3 6 000 as determined by sucrose density gradient ultracentrifugation amino acid analysis and gel filtration, respectively; its sedimentation coefficient was 2.7 -2.9; in fully activated form it migrated as a sharp prealbumin band by disc gel electrophoresis. The isoelectric point of PKA was approximately 4.4.The moderately anionic factor XIIa had an estimated molecular weight of 120 000. The cationic factor XIa and kallikrein were separated by gel filtration, the clot-promoting substance having a molecular weight of 170 000 --180 000, while kallikrein was found t o be about 9 6 000. A kinin-forming, BAEe-hydrolysing fraction with an elution volume greater than that of egg albumin was recovered and it is assumed that it represents a fragment of kallikrein.It is concluded that during massive surface adsorption, most of the adsorbed factor XIIa becomes fragmented. This fragment acquires prekallikrein-activating activity. The fragment retains some of the property of the parent molecule by being able to partially correct the clotting defect of factor XIIdeficient plasma. These findings are in good agreement with those obtained under similar conditions with human plasma.

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