Platelets stimulate tissue factor, the initiator of the extrinsic coagulation pathway, and increase fibrinolytic inhibition in fibroblasts grown in vitro. Cellular tissue factor increases an average of 2.8-fold over the control levels after a 6-hr incubation with platelets, and no activity is present in the media. Fibrinolytic inhibition is stimulated in both the fibroblasts and their media in the presence of platelets and accumulates throughout a 24-hr incubation. Neither leukocytes nor erythrocytes stimulate these changes. Both tissue factor and fibrinolytic inhibition increases are dependent on platelet concentration and are blocked by inhibitors of RNA or protein synthesis. Control smooth muscle cells have higher tissue factor and fibrinolytic inhibition than fibroblasts, but their response to the presence of platelets is similar. Confluent monolayers of endothelial cells have very low levels of tissue factor that are not altered by the presence of platelets. However, the ability of endothelial cells to inhibit fibrinolysis is enhanced by the presence of platelets. The fraction that stimulates tissue factor and fibrinolytic inhibition is distinct from platelet-derived growth factor and from the fraction that enhances leukocyte tissue factor. It is associated with an insoluble, nonmitogenic fraction that is not inactivated by phospholipase C, or diisopropylfluorophosphate, nor is it chloroform:methanol extractable. Platelets are a physiologic modulator for both cellular tissue factor and the fibrinolytic system in vitro.