Protein disulfide isomerase (PDI) is an abundant reticulum endoplasmic protein but also acts as an important functional regulator of some extracellular surface proteins. Recent studies suggest that PDI plays a role in integrin activation and thrombus formation. The aim of this study was to examine whether activation of integrin is the first stage leading to release of PDI from the subcellular compartments of endothelial cells to extracellular space. Our results show that endothelial cells which adhere to fibronectin or fibrinogen release significantly more PDI than those which adhere to poly-L-lysine. Cells treated with RGD peptide, Src and FAK kinase inhibitors and anti alphaVbeta3 antibody display lower PDI secretion. The destruction of the actin cytoskeleton of endothelial cells by cytochalasin D inhibits PDI release. When the endothelial cells adhere to fibrinogen or fibronectin, PDI and alphaVbeta3 gain free thiol groups. Our data suggest that upon activation of integrins, PDI is released from endothelial cells and forms a disulfide bond complex with alphaVbeta3 integrin.