PECAM-1–deficient platelets display normal platelet aggregation responses to G-protein–coupled agonists, PAR-4 peptide, ADP, and calcium ionophore, but are hyperresponsive to GPVI-specific agonist collagen-related peptide (CRP). Aggregation responses of PRP (platelet count adjusted to 100 × 10⁹/L) for wild-type and PECAM-1^–/– mice were determined following activation with different concentrations of various agonists. (A-C) PAR-4 agonist peptide used at 125, 250, and 500 μM, respectively. (D-F) ADP used at 2.5, 5, and 10 μM, respectively. (G-I) Calcium ionophore (CI) A23198 used at 1.25, 5, and 10 μg/mL, respectively. (J-L) CRP used at 1.25, 2.5, and 5 μg/mL, respectively.