Since the first publication of the production of monoclonal antibodies in 1975 there has been enormous interest in the possible medical applications. Monoclonal antibodies are now widely used in all areas of biological and medical research as well as in clinical diagnostic tests and in therapy. This review concentrates on the clinical use of antibodies in therapy particularly with regard to the properties of the antibodies which seem most relevant to their usefulness. In-vitro tests using human effector systems and in-vivo animal models have demonstrated the importance of the antibody isotype and valency for antigen as well as the specificity of binding. With the appropriate choice of antibody, used either alone, or conjugated to radioisotopes, drugs or toxins, it is possible to specifically deplete subpopulations of cells both in-vitro and in-vivo. Such antibodies can be used for tumour therapy, and for immunosuppression in organ transplantation and in autoimmune disorders.