Two flow cytometric parameters are generally used to quantify platelet activation as measured by P-selectin (CD62) expression: percentage and mean channel fluorescence of CD62-positive platelets (%(+) and MCF+, respectively). We describe a method for calculation of indices of platelet activation for positive (IPA(+)) and total (IPA(Sigma)) platelets, which reflect integrated amounts of CD62 expressed in these populations; IPA(+) is calculated as the product of %(+) and MCF+, whereas IPA(Sigma) is exclusively determined by mean fluorescence of the total platelet population (MCFSigma) and does not depend on %(+). We use these parameters to characterize human platelet activation in whole blood samples treated with varying human alpha-thrombin concentrations, mimicking the variations in platelet activation in a number of clinical settings. Multiparameter analysis of CD62 expression may be useful for selective diagnosis of disorders with systemic or localized platelet activation and for monitoring the clinical course of the disease and effect of therapeutic interventions.