We have analyzed the plasminogen activator (PA) systems of two metastatic breast adenocarcinoma cell lines, MCF-7 and MDA-MB-231, as a function of 17 beta-estradiol stimulation when the cells were cultured on purified components of extracellular matrix. Laminin enhanced PA levels in both cell lines, but this enhancement seemed to occur via different mechanisms, including dissociation of inhibitor complexes. The major effect was the marked increase in cell-associated urokinase-type PA (u-PA); the increase was independent of estrogen in hormone-insensitive MDA-MB-231 cells grown on laminin-coated surfaces. In estrogen-sensitive MCF-7 cells, 17 beta-estradiol stimulated u-PA secretion in a similar fashion on plastic, laminin, fibronectin, or collagen but acted in synergy with laminin in the production and release of tissue-type PA.