The present study examined the effects of the androgen steroid, dihydrotestosterone (DHT), on murine T-cell production of a number of lymphokines. Direct exposure of murine T cells to DHT in vitro was found to reduce the amount of interleukin-4 (IL-4), IL-5, and gamma-interferon (gamma IFN) produced after activation with anti-CD3 without affecting the production of IL-2. Exposure of T cells to either androstenedione or testosterone (the metabolic precursors of DHT) affected no change in the biosynthesis of either of these lymphokines. We have determined that macrophages possess 5 alpha-reductase, and are thus competent to metabolize testosterone to DHT. This physicochemical information is complemented by a functional analysis of macrophage metabolism of testosterone. By incubating bone marrow macrophages with testosterone, before their use as accessory cells, the IL-4 and IL-5 producing potential of the activated T cells cocultured with them was depressed. That the observed effect was mediated by the conversion of testosterone to DHT was further corroborated by illustrating that the inhibition of IL-4 production was abrogated if 4MA, a specific 5 alpha-reductase inhibitor, was added to macrophage cultures containing testosterone. The biologic role of DHT in lymphokine and immune response regulation in vivo was addressed using several lines of investigation. First, transdermal delivery of DHT to groups of mice altered the capacity of T cells residing in the draining lymph nodes, only, to produce lymphokines. Second, treatment of either aged mice or the T cells isolated from them with a combination of dehydroepiandrosterone and DHT restored the capacity of their T cells to produce IL-2, IL-4, and gamma IFN to levels equivalent to that of younger mice. Finally, we observed a difference between males and females of a given age to produce IL-2, IL-4, and gamma IFN, with both IL-4 and gamma IFN production being elevated in females. Collectively, our findings indicate that DHT, similar to other steroid hormones, may play an important role in lymphokine regulation in vivo.