This review highlights the properties, problems, and potentials of monoclonal antibodies as diagnostic and therapeutic agents. The most extensive experience has been obtained with antibodies specific for functionally distinct subsets of lymphocytes. They have been used to monitor the immunosuppression of organ-graft recipients and to attempt to elucidate the disturbance of immunologic function in a variety of conditions. Current therapeutic applications under trial include immunosuppression to treat organ-graft rejection and to eliminate cells responsible for graft-vs-host disease from the bone marrow. Applications to cancer diagnosis and treatment have been hampered by the difficulty of identifying truly tumor-specific antigens. Successes have, however, been obtained in the location of metastases and in the extracorporeal treatment of autologous marrow to purge it of malignant cells, and, more rarely, with the direct administration of monoclonal antibodies in vivo. The conjugation of cell-type specific monoclonal antibodies with cytotoxic agents should overcome a number of limitations.