It has been found that Hageman factor is adsorbed from normal human plasma by collagen and elastin. The adsorption is dependent upon the concentration of both substances and upon the adsorption time. The adsorption by collagen is almost selective. By the procedure of the repeated adsorption it is possible to obtain plasma containing 3-5% of Hageman factor normal value and normal amount of PTA (factor XI). At pH 9.5-10.0 the Hageman factor can be eluted in part from collagen shaken previously with human or pig plasma, its recovery in the eluates from elastin is poor. Experimental evidence is presented that the Hageman factor is activated following adsorption on collagen. The eluates from collagen have been compared with those prepared in a similar manner from kaolin and celite. All eluates possess clot promoting, TAMe arginine esterase and fibrinolytic activities. They shorten considerably the clotting time of the whole blood in siliconized tubes. It has been suggested that collagen may act as an intrinsic trigger of the blood coagulation in vivo by adsorbing and activating the Hageman factor. * Aided in part by the Polish Academy of Sciences, Department VI.