13-trans retinoic acid (13-trans RA) is an effective inducer of differentiation of acute promyelocytic (APL) cells both in vivo and in vitro. It is used in the induction of remission of patients with APL. We found, by using the promyelocytic NB4 cell line established from a patient with APL, that the induction of differentiation with RA was accompanied by modulation of the plasminogen activation system. The expression of urokinase (uPA) activity was rapidly increased in the growth medium and at the surface of cells treated with RA. The high uPA activity was counteracted both in the growth medium and at the cell surface by an increased plasminogen activator inhibitor (PAI) production and reduction of uPA synthesis. The expression of uPA receptor and PAI-2 were stimulated and persisted at 48 hours from RA addition. The simultaneous induction of CD11b suggests that differentiation results in increased production of both. APL patients often encounter episodes of disseminated intravascular coagulation that are associated with secondary fibrinolytic events. Our results suggest that downregulation of uPA activity results in the decrease of plasmin on the surface of the differentiated cells, which may reduce the occurrence of fibrinolytic episodes of patients with APL.