1. By use of whole cell patch-clamp and Indo-1 fluorescence studies of the Jurkat T leukaemic cell line, we show that the new organic antagonist of receptor-mediated Ca2+ entry, SK&F 96365, inhibits the T cell Ca2+ current in a dose-dependent fashion, with an IC50 of 12 microM. 2. SK&F 96365 also inhibits [3H]-thymidine incorporation and interleukin-2 (IL-2) synthesis in peripheral blood lymphocytes. 3. SK&F 96365 has no effect on Ca2+ stores release or K+ channels. 4. This is the first account of an organic inhibitor of the T cell Ca2+ current. The ability of SK&F 96365 to inhibit IL-2 synthesis and cell proliferation suggests that a new class of related Ca2+ channel blockers can be developed as immunosuppressive agents.