The Advia 120 Hematology System provides new platelet parameters that have been proposed as useful markers of platelet activation. The aim of this study was to investigate platelet parameter variations after adenosine diphosphate (ADP) activation of platelet-rich plasma (PRP) in vitro, with particular interest in the mean platelet component (MPC), which was compared with two well-known degranulation antigens. Changes in platelet parameters that were induced by the activation of PRP with different concentrations of ADP were examined first. The time course of parameter values up to 60 minutes after maximal ADP activation and the relationships between the MPC and P-selectin and granulophysin expression as determined by flow cytometry were then investigated. After 10 minutes of ADP stimulation, the MPC presented a dose-dependent increase. At the maximal ADP concentration, the initial increase of the MPC was followed by a progressive decrease, leading the MPC to become significantly lower with respect to the baseline after 60 minutes of incubation. Significant variations in other parameters are also described. Finally, a negative correlation was found between the MPC change with respect to time 0 and both P-selectin and granulophysin expression. The present study suggests that platelet parameter variation, particular in the MPC, may be used to assess platelet activation in PRPs stimulated by ADP.