FTY720, a synthetic myriocin analogue derived from culture filtrates of Isaria sinclairii, is a novel immunosuppressant that in experimental animals, nonhuman primates, and renal transplant recipients produces lymphocytopenia and prolongs allograft survival in a dose-dependent fashion. Furthermore, the synergistic interactions of FTY720 promote the immunosuppressive effects of calcineurin antagonists and/or proliferation signal inhibitors, providing the possibility of reducing exposure to and thereby ameliorating the toxicity of these drugs.