REJECTION continues to be the single largest impediment to successful organ transplantation¹. Antilymphocyte globulin, which contains antibodies that react with the leukocyte common antigen known as CD45 (refs 2–6), has proved to be one of the most effective agents for preventing rejection. We have shown earlier that a monoclonal antibody directed against the RB isoform of CD45 substantially inhibits the alloreactivity of human CD4⁺ lymphocytes in vitro⁷. Here we investigate whether CD45RB could be an appropriate target for preventing renal allograft rejection in mice. Mice treated with two injections of a monoclonal antibody (MB23G2) (ref. 8) raised against CD4SRB protein all survived and had normal renal function. Furthermore, this antibody reversed acute rejection when therapy was delayed until day 4, and the mice survived for their natural lifespan. The immunosuppression achieved may find application in the pre-vention and treatment of transplant rejection in man.