Platelet aggregation, induced by agonist-mediated activation of membrane glycoprotein (GP) IIb/IIIa, and binding of fibrinogen to GPIIb/IIIa, is commonly analyzed using an aggregometer in the clinical laboratories. However, this method has a limitation to get precise results on the samples with small number of platelet (less than 100,000/1) or hyperlipidemia. Recently, flow cytometry has been used to evaluate platelet function due to the detection of fibrinogen binding to activated platelets using fluorescence labeled fibrinogen or anti-fibrinogen antibody. However, the appropriate rule for evaluation of the results has not been established yet. We converted a ratio of fibrinogen binding platelets to a velocity per unit concentration of ADP as follows: a difference of two ratios of fibrinogen binding platelets on neighboring two ADP concentrations was divided by a difference of ADP concentrations. It was considered to be a mean velocity between the two ADP concentrations. We adopted the range of ADP concentration, which gave the maximum velocity, as an index of platelet activation. If the peak of maximum velocity move toward lower or higher ADP concentration, it means hyper- or hypoactivation of the platelets, respectively. The objectivity of this method may make it a useful technique for clinical examination of platelet function.