Cyclosporine, a cyclic endecapeptide of fungal origin, has recently been released for use in clinical transplantation. Trials in kidney, heart, liver and bone marrow recipients were encouraging: 1-year graft survival rates were 70% to 80% for kidney and heart recipients, and 60% to 65% for liver allograft recipients. Cyclosporine is also effective in treating bone marrow recipients with acute graft-versus-host disease. The drug selectively inhibits T-helper cell production of growth factors essential for B cell and cytotoxic T-cell differentiation and proliferation, while allowing expansion of suppressor T-cell populations. Drug absorption varies greatly, necessitating monitoring of drug level and individualization of therapy. Nephrotoxicity is the most frequent side effect of cyclosporine. An increased incidence of B-cell lymphomas seen when cyclosporine was used in conjunction with cytotoxic agents or anti-lymphocyte globulin has very rarely been observed when concomitant immunosuppression has been limited to low-dose corticosteroids. Lower initial doses of cyclosporine, followed by more rapid tapering may reduce the incidence of nephrotoxicity without compromising improved graft outcome.