Ornithine decarboxylase (ODC) is the initial enzyme in polyamine synthesis. An increase in ODC activity is associated with increased RNA, DNA, and protein synthesis. We have used the induction of ODC by mitogens and alloantigens in human peripheral blood lymphocytes as an intracellular marker of protein synthesis and lymphocyte activation. The immunosuppressive agent cyclosporine was found to inhibit both the mitogen and alloantigen stimulated induction of ODC in lymphocytes in a manner that parallels inhibition of subsequent 3H-thymidine incorporation. When purified T lymphocytes were stimulated with mitogen alone, minimal ODC activity was detected. The addition of 5% monocytes, human Interleukin-1 (IL-1), or T cell growth factor (IL-2) enhanced mitogen-induced ODC activity in T lymphocytes 4-10-fold. Cyclosporine inhibited the induction of ODC when T lymphocytes were combined with monocytes or growth factors. We conclude that (1) the induction of ODC in human lymphocytes by mitogen and alloantigen is inhibited in the presence of cyclosporine; (2) the induction of ODC activity in purified T lymphocytes requires the presence of both mitogen and monocytes or their products; (3) IL-1 and IL-2 can supplement for monocytes and augment the phytohemagglutinin induction of ODC in T lymphocytes; and (4) cyclosporine inhibits ODC induction in T lymphocytes stimulated with mitogen in the added presence of monocytes, IL-1, or IL-2. The inhibition of ODC induction and polyamine synthesis by cyclosporine adds insight into its mode of action on the mechanisms involved in early T cell activation.