The immunosuppressant cyclosporine A revolutionized treatment of graft rejection. Two newer agents, FK506 and rapamycin, show great clinical potential. These drugs suppress the immune system by forming protein-drug complexes that interact with and inhibit key components of the signal transduction pathways required for T-cell activation. The target of the cyclophilin A-cyclosporine A and FKBP12-FK506 complexes is calcineurin, a protein phosphatase required for signaling via the T-cell receptor. Cyclosporine A and FK506 nephrotoxicity may reflect renal-specific functions of calcineurin. The target of the FKBP12-rapamycin complex is TOR, a lipid and protein kinase homolog that is likely to be required for T-cell proliferation in response to interleukin-2. The identification of cyclosporine A, FK506, and rapamycin targets reveals much concerning T-cell signaling and provides the means to design novel immunosuppressants with reduced toxicity.