Complement-mediated immune thrombocytopenia results in delayed thrombopoiesis in PECAM-1^−/− mice. Two- to 4-month-old PECAM-1^−/− mice (n = 8) and PECAM-1^+/− mice (n = 8) with littermate controls (n = 8) were given a sterile intraperitoneal injection of anti–mouse GP1bα antibody (2 μg/g of mouse). Platelet counts were measured at 48, 72, 120, and 172 hours after injection. *P < .05; **P < .005. Error bars indicate standard error.