Washed platelet suspensions contain active SF (surface factor or activation product), when prepared from normal (N) subjects or from patients deficient in factors Vili, IX or X, but not from a Hageman-deficient. Factor V-deficient plasmas were normalized by platelets from N, XII- and other subjects. Allowing for (a) the platelet prothromboplastic phospholipids, and (b) the above SF and AcG (V) effects, there was no evidence of any significant factors Vili, IX or X in the platelet plasmatic atmosphere. Since XII-platelets showed no true correction of plasmas deficient in Vili, IX or X, the minor “improvement” of test results in these (and N) plasmas on adding N, Vili-, IX- or X-platelets can be explained as a nonspecific surface factor effect. XII-platelets when incubated with normal plasma were normalized by adsorbing Hageman factor (XII) on their surface. * This study was aided by USPHS Research Grant No. HE-01510-10, and NIH Institutional Research Grant HE-06350-02 for Thrombosis Research.