White blood cell infiltration across an activated brain endothelium contributes to neurologic disease, including cerebral ischemia and multiple sclerosis. Identifying mechanisms of cerebrovascular activation is therefore critical to our understanding of brain disease. Platelet accumulation in microvessels of ischemic mouse brain was associated with endothelial activation in vivo. Mouse platelets expressed interleukin-1α (IL-1α), but not IL-1β, induced endothelial cell adhesion molecule expression (ICAM-1 and VCAM-1), and enhanced the release of CXC chemokine CXCL1 when incubated with primary cultures of brain endothelial cells from wild-type or IL-1α/β-deficient mice. A neutralizing antibody to IL-1α (but not IL-1β) or application of IL-1 receptor antagonist inhibited platelet-induced endothelial activation by more than 90%. Platelets from IL-1α/β-deficient mice did not induce expression of adhesion molecules in cerebrovascular endothelial cells and did not promote CXCL1 release in vitro. Conditioned medium from activated platelets inducedan IL-1α-dependent activation of mouse brain endothelial cells and supported the transendothelial migration of neutrophils in vitro. Thus,wehave identified platelets as a key source of IL-1α and propose that platelet activation of brain endothelium via IL-1α is a critical step for the entry of white blood cells, major contributors to inflammation-mediated injury in the brain. © 2010 by The American Society of Hematology.