Abstract Because previous studies show that lipoproteins affect platelet aggregation, we studied the effect of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) on the binding of fibrinogen, which mediates platelet-platelet contact. Neither LDL nor HDL induced 125I-fibrinogen binding at concentrations up to 2 g protein/L. In contrast, platelets stimulated with 10 μmol/L ADP bound 63 734±2453 molecules of fibrinogen per platelet. A 5-minute preincubation with LDL (0.5 to 2 g/L protein) induced a dose-dependent increase to 91 3O7±2164 molecules of fibrinogen per platelet at 1.5 g/L, which is in the range found after optimal stimulation with a-thrombin. The increased fibrinogen binding in the presence of LDL resulted in faster aggregation with a 16% increase in single platelet disappearance and a faster optical aggregation at 5 μmol/L ADP and 1.5 g protein/L LDL. Inhibition of prostaglandin G2/H2-thromboxane A2 formation with indomethacin (30 μmol/L) did not change the stimulation by LDL. In contrast, modification of h/sine residues of LDL, which is known to prevent specific binding to platelets, completely abolished the effect of LDL. Under the same conditions HDL did not change fibrinogen binding or aggregation. LDL also enhanced α-thrombin-induced [12514C]serotonin secretion, but this property was not affected by h/sine modification of LDL. These data indicate that LDL enhances platelet aggregation by stimulating the mechanisms that control exposure of fibrinogen binding sites on the gh/coprotein IIB/IIIA complex via a mechanism that differs from the effect of LDL on secretion.