Three-day, concanavalin A-induced T lymphoblasts have been used as a model to study lymphokine release from sensitized T cells. The blasts responded to interleukin 2 (IL-2) but did not constitutively produce this or other lymphokines. After mitogen restimulation, blast cells synthesized IL-2 as well as gamma-interferon, B cell-stimulating factor(s), and cytolytic differentiation factor(s). This production resulted from the induction of biologically active lymphokine mRNA. Cyclosporin A (CSA), a potent immunosuppressive agent, strongly inhibited synthesis of IL-2, gamma-interferon, and B cell- and CTL-stimulating factor(s), from mitogen-restimulated T blasts. In contrast, CSA did not block the cytolytic activity of the T blasts, nor modify bulk protein synthesis induced by Con A. CSA also blocked lymphokine release from a phorbol myristate acetate-stimulated thymoma cell line, EL-4. The effect of CSA was to block the induction of active lymphokine mRNA, as assayed in an oocyte translation system. This selective inhibition of lymphokine mRNA suggests that CSA may be useful in the therapy of inflammatory, lymphokine-mediated disease states.