Pleiotrophin is an 18 kDa secreted polypeptide growth factor with direct pro-angiogenic and tumorigenic properties. Pleiotrophin is a substrate for proteolytic enzymes, such as plasmin, leading to proteolytic fragments with distinct activities on endothelial cell activation in vitro or angiogenesis in vivo. Aprotinin is a naturally occurring broad spectrum protease inhibitor, used widely in cardiac surgery due to its ability to inhibit plasmin and reduce perioperative bleeding. Since we have seen that aprotinin inhibits proteolysis of pleiotrophin by plasmin, the aim of the present study was to evaluate the possible role of pleiotrophin in the effects of aprotinin on angiogenesis and human endothelial cell migration. Our data demonstrate that aprotinin, in a concentration-dependent manner, is angiogenic in the chicken embryo chorioallantoic membrane assay in vivo and induces human endothelial cell migration in vitro. Aprotinin inhibits pleiotrophin proteolysis and induces expression and secretion of pleiotrophin through an AP-1-dependent transcriptional activation of the pleiotrophin gene, and pleiotrophin seems to mediate the stimulatory effects of aprotinin on cell migration through its receptor protein tyrosine phosphatase beta/zeta. The stimulatory effect of aprotinin on pleiotrophin expression and cell migration may explain, at least partly, the problems observed with the clinical use of aprotinin.