Muromonab CD-3 (OKT-3) is a monoclonal antibody that is highly effective in the treatment of acute rejection in solid organ transplants. Due to its monoclonal nature, each molecule is identical because it is derived from a single antibody-producing clone. OKT-3 is administered only by intravenous injection and has a harmonic half-life of approximately 18 hours. It binds specifically to the CD-3 complex, which is involved in antigen recognition and cell stimulation, on the surface of T lymphocytes. Immediately after administration CD-3-positive T lymphocytes are abruptly removed from the circulation. The route of metabolism for OKT-3 is not clear; it may be removed by opsonization by the reticuloendothelial system when bound to T lymphocytes, or by human antimurine antibody production. The agent has been effective in reversing corticosteroid-resistant acute rejection in renal, liver, and cardiac transplant recipients. Its use in pancreatic and bone marrow recipients is inconclusive. OKT-3 has a considerable number of initial side effects, and some life-threatening reactions may occur. This drug should not be administered to any patient who is greater than 3% usual body weight because of the potential for the development of severe pulmonary edema. OKT-3 may also be associated with a high rate of infection, especially of the viral type. The usual dose is 5 mg administered as an intravenous bolus over 2-4 minutes daily for 10-14 days. Approximately 85% of patients treated with OKT-3 develop reactive human antimurine antibodies that, over time, may lead to tachyphylaxis and neutralization of the murine antibody OKT-3. OKT-3 is potent immunosuppressive agent and is an important prototype of future monoclonal antibodies.