Ionizing radiation induces the inflammatory response in part through leukocyte binding to cell adhesion molecules that are expressed on the vascular endothelium. We studied the effects of X radiation on the pattern of immunohistochemical staining of CD62P (P-selectin). P-selectin was localized within cytoplasmic granules in the untreated vascular endothelium. Immunohistochemical staining of P-selectin was observed at the luminal surface of vascular endothelium within 1 h of irradiation. Increased P- selectin staining at the blood-tissue interface occurred primarily in pulmonary and intestinal blood vessels. To determine whether localization of P-selectin at the vascular lumen occurs through exocytosis of endothelial cell stores in addition to platelet aggregation, we removed the vascular endothelium from the circulation and irradiated endothelial cells in vitro. In this system, we studied the mechanisms by which ionizing radiation induced translocation of P-selectin by using immunofluorescence of human umbilical vein endothelial cells (HUVEC) and confocal microscopy. Prior to irradiation, P-selectin is localized in cytoplasmic reservoirs of HUVEC. After irradiation of HUVEC, P-selectin was translocated to the cell membrane, where it remained tethered. The lowest dose at which we could expect translocation of P- selectin to the cell membrane was 2 Gy. To determine whether P-selectin in Weibel-Palade bodies requires microtubule-dependent membrane transport, we added two microtubule-depolymerizing agents, Colcemid and nocodazole. Microtubule-depolymerizing agents prevented radiation-induced translocation of P-selectin to the cell membrane. Thus P-selectin accumulates in irradiated blood vessels through both platelet aggregation and microtubule-dependent exocytosis of storage reservoirs within the vascular endothelium.