The molecular mechanisms which govern the biosynthesis and secretion of the various T cell-derived lymphokines are poorly understood at this time, in spite of their tremendous importance to the control of the mammalian immune system. Here we provide compelling evidence that production of the murine T cell growth factors interleukin (IL) 2 and IL4 are differentially regulated by glucocorticoid (GCS) hormones. Under conditions where IL2 production is reduced by GCS hormones, IL4 production is increased. In vivo, this effect on T cell production of growth factors is manifest at low GCS concentrations that are well within physiologic ranges. In vitro, splenocytes isolated from antigen-stimulated donors, as well as antigen-specific cloned T cell lines, undergo alterations in their capacity to secrete T cell growth factors when stimulated with antigens in the presence of GCS. Responses normally dominated by IL2 are dramatically shifted to a condition where IL4 represents the major species of T cell growth factor produced. Similar changes in the pattern of T cell growth factor production are observed following short pulses with low-dose GCS in vitro, and the steroid-induced depression in IL2 production can be reversed and/or inhibited by treatment with the potent steroid antagonist RU486. Our results imply that GCS hormones, presumably through their capacity to activate a specified family of ligand-dependent transcriptional regulatory proteins (steroid hormone receptors), function to control the pattern of lymphokines produced by activated T cells. Steroid-mediated regulation of lymphokine gene expression could serve to dictate the types of immune effector mechanisms which can be initiated subsequent to antigen exposure.