Intact lipoprotein (a) (Lp (a)) is known to decrease collagen-stimulated platelet aggregation in vitro, though the nature of the interaction between this lipoprotein and stimulated platelets is unknown. Lysine binding regions of Lp (a) facilitate binding between it and at least one cell type. Epsilon aminocaproic acid (EACA) renders Lp (a)'s lysine binding regions incapable of cellular interaction using those regions. Washed human platelets were presented vehicle alone or with varying concentrations of Lp (a) either not exposed or previously exposed to increasing levels of EACA. In all experiments, EACA failed to affect the impact of Lp(a) on platelet aggregation. It is concluded that the lysine binding regions of Lp (a) do not mediate the intact Lp(a)-induced reduction of collagen-stimulated platelet aggregation in vitro.