Antiplatelet therapy with glycoprotein IIb-IIIa inhibitors reduces the incidence of cardiac events in patients with acute coronary syndromes. A lack of universal efficacy may result from interindividual variation in the inhibition of fibrinogen binding after exposure to tirofiban and eptifibatide. Accordingly, accurate monitoring of platelet function in individual subjects may be needed. To assess this possibility, blood was drawn from 15 healthy volunteers into syringes containing corn trypsin inhibitor (an anticoagulant that is a specific inhibitor of factor XIIa) and selected concentrations of tirofiban and eptifibatide. Platelets were then activated with adenosine diphosphate (ADP) and thrombin receptor agonist peptide. Flow cytometry was used to assess activation with respect to glycoprotein IIb-IIIa activation as reflected by fibrinogen binding and alpha-granule degranulation as reflected by P-selectin expression. In platelets activated with 1 microM ADP, clinically relevant concentrations of tirofiban caused inhibition of fibrinogen binding ranging from 17% to 88%. Similarly, eptifibatide caused inhibition of fibrinogen binding ranging from 32% to 74%. The highest concentration of eptifibatide tested enhanced agonist-induced degranulation, an effect not seen with tirofiban at concentrations tested. Flow cytometry in minimally altered whole blood can discriminate variation in the response to glycoprotein IIb-IIIa inhibitors with respect to specific components of platelet activation. Thus, the approach developed should facilitate definition of optimal platelet inhibition and individualized tailoring of therapy to induce optimal effects.