Binding of PAR3 IgG to the surface of mouse platelets by flow cytometry. Washed platelets from C57BL/6J mice were incubated with preimmune or PAR3 IgG as indicated and then washed and incubated with FITC-goat antirabbit IgG. After three washes, platelets were analyzed by flow cytometry. Each figure represents an analysis of 10,000 events with SSC (side scatter) on the abscissa and FITC fluorescence intensity on the ordinate. The median FITC fluorescence intensities were 3.5 for preimmune IgG and 27.4 for PAR3 IgG. The same population of cells that bound PAR3 IgG bound antibody to the platelet marker CD41; PAR1 IgG showed no significant binding (not shown).