Quantitative flow cytometry of platelet glycoproteins. Platelet surface glycoproteins were measured in platelet-rich plasma (PRP) using specific mouse monoclonal antibodies by flow cytometry. Platelets were stained with a glycoprotein-specific antibody (10 μg/mL) for 15 minutes and then an FITC-labeled antimouse antibody was added and incubated for a further 15 minutes in the dark. Simultaneously, calibration beads were incubated with the antimouse FITC secondary for 15 minutes. Diluent was added to all tubes, which were analyzed by flow cytometry. The calibration curve was obtained by plotting the geometric mean fluorescence intensity (GMFI) for each peak of the histogram (M1,M2,M3, and M4 in the left panel) against the known number of antibody binding sites for that peak (right panel). The value of the GMFI for the isotype control is always subtracted from the GMFI of the specific antibody binding before the number of specific sites is calculated.