In the circulation, adherent thrombi are most likely formed by nucleation and growth processes. The adsorption of an anchoring layer must be followed by further constituent interactions; these by crosslinking introduce cooperative effects which affect nucleation and growth rates. Adsorption of thrombin to nonpolar surfaces has been examined using a clotting time assay. Assay discrimination was determined by coefficients of variation: of the assay itself (0.6Oj,), of reproducing diluted thrombin sources (0.37%), and of reproducing sets of clotting tubes containing fibrinogen (1 5%). Adsorptions are 0.54 mg/m2 at the solution-air interface, 0.77 mg/m2 a t the solution-clean wall interface, and 0.04 mg/m2 to walls previouslx treated with thrombin. Average distance between thrombin molecules is 89 A, the minimum adsorption energy is -11.7 kcal/mole and estimated thrombus adhesive strength 10 kg/cm2. Thrombin can be the anchoring layer for a thrombus which contains thrombin, fibrin and platelets since all constituents interact with each other. The thrombin-fibrin interaction energy is about -7 kcal/mole. The platelet can, a t least on interaction with thrombin, undergo structural changes which increase interaction. A variety of interactions and cooperative effects are possible. A problem is t o develop protocols and assays which will permit specification of the anchoring layer and characteristics of subsequent interactions.