The effect of RU 486, a synthetic steroid that is a powerful antagonist of glucocorticoid hormones, was tested on the transcription of several glucocorticoid-regulated genes in different cell types: inflammatory murine macrophages and two human mammary glandderived cell lines, MDA-MB-231 and HBL-100. The transcription of genes which are positively regulated by glucocorticoids (e.g., tissue-type plasminogen activator and c-myc in mammary cells, c-fos in macrophages) and that of genes which are negatively regulated by these agents (e.g., urokinase-type plasminogen activator in all three cell types, TNF-a and IL-1 in macrophages) was explored. RU 486 almost completely prevented the effects of dexamethasone on the transcription of these various genes. When added alone, RU 486 had essentially no agonist activity.