The effects on fibrinolysis of purified normal human plasma lipoproteins and their apolipoproteins (apo) were assessed in an in vitro system containing urokinase, plasminogen, and fibrin. High and very low density lipoproteins (HDL and VLDL, respectively) but not low density lipoproteins (LDL) significantly increased lysis of radiolabeled fibrin or lysis area of plated fibrin compared to controls. Apo HDL, apo HDL2, apo HDL3, apo VLDL (buffer soluble apo), apo AI and AII activated fibrinolysis by 1.3-1.4 fold of control values, whereas apo B (as a narrow density cut of LDL) and non-lipoprotein control proteins (albumin and myoglobin) did not. Addition of HDL in the absence of urokinase failed to activate fibrin lysis. These data demonstrate that the major proteins of HDL - apo AI and apo AII - and possibly certain minor constituent(s) common to HDL and VLDL participate in the fibrinolytic process.