Cyclosporine is a fungal metabolite increasingly used as an immunosuppressive agent in organ transplantation. It provides immunosuppression primarily through reversible inhibition of T-lymphocytes, without myelotoxicity. Cyclosporine appears to inhibit primary activation of helper cells and to decrease the production of lymphokines by these cells. Cyclosporine becomes widely distributed into tissue compartments and is metabolized primarily by the liver. Serum trough levels, measured by radioimmunoassay or high pressure liquid chromatography, are commonly monitored and sometimes used for dose adjustments. Cyclosporine has been used in human renal, cardiac, liver, bone marrow, and pancreatic transplantation, as well as in other experimental animal models. It has a narrow therapeutic index, with major complications arising from nephrotoxicity, hypertension, and hepatotoxicity. Cyclosporine also has some antischistosomal and antimalarial activity.