The immune system is regulated by the interactions among several distinct functional subsets of T cells. The action of several commonly used immunosuppressive drugs on the activation of cytotoxic T lymphocytes (CTL) and suppressor T lymphocytes (Ts) in the primary mixed lymphocyte reaction (MLR) was investigated. Cyclosporin A, hydrocortisone, and azathioprine were all found to inhibit both CTL and Ts activation when present at pharmacological doses in culture. When these drug-inhibited cultures were reconstituted with interleukin-2, however, clear differences between the effects of these drugs was observed. Cyclosporin A and hydrocortisone allowed the selective activation of Ts in the presence of interleukin-2, while azathioprine inhibition was not reversed by interleukin-2. Thus, CTL precursors appear to be directly inhibited by all of these drugs, but Ts precursors apparently are not inhibited by cyclosporin A or hydrocortisone provided interleukin-2 is present. These findings are discussed in terms of the activation requirements of CTL vs. Ts and the implications of the selective activation of alloantigen-specific Ts for prevention of allograft rejection.