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The aim of this work was a simultaneous analysis of functional platelet activity using both standard optical aggregometry and flow cytometry, by means of measuring P-selectin and GP IIb-IIIa glycoprotein receptor contents on the surface of ADP-induced platelets. GP IIb-IIIa amounts and P-selectin expression on cell surface are significantly increased on platelets after activation with 10 mcM ADP, both in normal donors and in aspirin-treated patients. In clopidogrel-treated patients, the amounts of expressed P-selectin and GP IIb-IIIa did not change after ADP induction, thus suggesting absence of detectable ADP-induced platelet activation in this group. By means of Spearman criterion, a distinct correlation was found between the P-selectin expression, GP IIb-IIIa contents, and rates of standard ADP-induced aggregation. A combination of standard ADP-induced aggregation/activation of platelets with flow cytometry using FITC or PE-labeled antibodies to P-selectin and GP IIb-IIIa, allowed us to apply immunological approaches to analysis of platelet hemostasis, to propose novel laboratory criteria for evaluation of platelet hyperaggregation, and to detect differential effects of clopidogrel and aspirin.