PECAM-1–deficient platelets display normal alpha secretion. Surface expression of P-selectin was determined for washed platelets stimulated by 0 to 2.5 U/mL thrombin or 100 to 500 μM PAR-4 agonist peptide and then stained with either a buffer control and FITC–P-selectin mAb for both wild-type and PECAM-1^–/– platelets. FITC-labeled samples were analyzed on a FACSCalibur analyzer. Results are cumulative data from 3 independent experiments and are presented as geometric mean fluorescence intensity (GMFI) ± SEM.