Apoprotein(a), (apo[a]), the specific antigen of lipoprotein(a) (Lp[a]), consists of structural domains (a serine protease unit, kringles 4 and 5) with marked homology to those of the corresponding domains in plasminogen. In this study, we have investigated the impact of this unique structural mimicry on the binding and activation of plasminogen by fibrin-bound tissue-type plasminogen activator at the plasma-fibrin interface. We found that the total amount of plasmin generated on the surface of fibrin was decreased in the presence of high concentrations of Lp(a): 197 +/- 65 fmol in plasmas with greater than 60 mg/dl Lp(a) versus 287 +/- 112 fmol in control plasmas. A similar effect was also apparent in the corresponding euglobulin fractions (554 +/- 169 fmol versus 754 +/- 310 fmol), the latter lacking the plasminogen-binding proteins alpha 2-antiplasmin and histidine-rich glycoprotein, but containing Lp(a). The difference between plasma samples was significant (p less than 0.05) as calculated from the percent decrease in plasmin generated from plasmas with high levels of Lp(a) relative to that generated in the paired controls with low Lp(a) levels. The involvement of Lp(a) was verified in a reconstituted system consisting of normal human plasma supplemented with 100 mg/dl of either purified Lp(a) or low density lipoprotein. Lp(a) produced a decrease of 30% in the generation of plasmin (180 fmol versus 255 fmol in plasma, and 485 fmol versus 705 fmol in the euglobulin fraction). Moreover, using a radiolabeled sheep antibody against human apo(a), we were able to demonstrate the binding of 40 fmol Lp(a) to fibrin during ongoing plasminogen activation. These results indicate that Lp(a) impairs the binding of plasminogen to fibrin and thereby decreases the generation of plasmin by occupying C-terminal lysine residues unveiled on the fibrin surface by plasmin degradation as recently reported (Circulation 1990;82[suppl III]:III-92). In consequence, impairment of fibrinolysis and accumulation of Lp(a) at sites of vascular injury may occur, factors that may be important in the development of atherosclerosis and associated thrombosis.