Most plasma proteins are present in both normal and atherosclerotic intima, and their concentrations in intimal interstitial fluid are directly related to plasma concentration and molecular size. All intimal samples also contain soluble fibrin-fibrinogen-related antigens, consisting of variable mixtures of fibrinogen and fibrinogen and fibrin degradation products; the extracted washed tissue contains insoluble fibrin. It appears that the fibrinogen is subjected both to degradation and to conversion to fibrin, which in turn undergoes lysis. Biochemically, insoluble "fibrin" can be detected by incubating the washed tissue with plasmin, and assaying the fibrin degradation products that are released; they are released from all samples, including small amounts from normal intima. The fibrin could arise by incorporation of a mural fibrin clot and/or clotting of the fibrinogen within the intima, which contains a "cocktail" of clotting-related factors including prothrombin/thrombin-related antigens, antithrombin III (AT III), and other protease inhibitors: in recent experiments stripped intima was immediately treated with ethylenediaminetetraacetic acid, minced, extracted with Tris-buffered saline, and the extracts analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with antisera to prothrombin and AT III. Surprisingly, all blots treated with antiprothrombin antisera showed large bands migrating with free thrombin (36 to 37 kDa) and prothrombin (74 kDa) and four intermediate bands. In addition, in all 10 samples examined a band of high molecular mass (170 kDa) but variable intensity was present. This 170-kDa band comigrated with the major band reacting with anti-serum to AT III. The theoretical 1:1 thrombin:AT III complex (98 kDa) was not detected. Thus active thrombin appears to be present in intima, and this may be a highly atherogenic factor, both causing fibrin deposition and acting as a potent mitogen for arterial smooth muscle cells.