During immune responses, activated endothelial cells down‐regulate thrombomodulin and up‐regulate tissue factor expression leading to the development of a procoagulant surface. CD4⁺ T cells are known to promote endothelial cell procoagulant activity, however, the molecular interactions that mediate this effect are not completely known. CD40L is an activation‐induced CD4+ T cell surface molecule that functionally interacts with CD40 expressed on endothelial cells. In this study we ask if CD40L‐CD40 interactions modulate endothelial cell surface tissue factor or thrombomodulin expression in vitro. Human umbilical vein endothelial cells (HUVEC) were cocultured with control cells or CD40L⁺ Jurkat T cells in the presence or absence of anti‐CD40L mAb. By two‐color FACS analysis we demonstrated that CD40 ligation induces HUVEC tissue factor expression and thrombomodulin down‐regulation. Utilizing neutralizing antibodies, we show that CD40L‐mediated tissue factor and thrombomodulin modulation, as well as E‐selectin and VCAM‐1 up‐regulation, is independent of tumor necrosis factor α, interleukin‐1α, or interleukin‐1β production. Together these data suggest that CD40L‐CD40 interactions may directly regulate endothelial cell procoagulant activity during inflammatory responses. J. Leukoc. Biol. 63: 373–379; 1998.