Cyclosporine is a potent new immunosuppressive agent which is now available for use in clinical transplantation. It has a unique mechanism of action, interfering primarily with the cell-mediated response to foreign antigens. Cyclosporine has been very effective in preventing early graft loss due to rejection and has resulted in improved graft survival following kidney, heart, and liver transplants. Its major disadvantage is nephrotoxicity, which may be early (due to synergistic effects with other problems such as ischemic renal failure) or long-term (interstitial fibrosis and tubular atrophy). As cyclosporine is used more effectively, graft survival may be improved with minimal risk of toxicity.