Antibodies to cultured human lymphoblasts were raised in horses using a schedule employing both subcutaneous and intravenous routes of injection. Plasma from groups of horses was pooled and the IgG prepared from each pool was tested extensively for safety and immunosuppressive efficacy in vitro and in vivo. On the basis of the results of skin grafting in monkeys, only globulins derived from the first main bleeds were blended to produce a bulk for clinical use. One early pool of globulin was discarded because when undiluted, it was lethal in monkeys by the intravenous route, and another pool was discarded because it contained antibodies which bound to the glomerular basement membrane of rats. No signs of toxicity as judged by haematology, blood biochemistry, and histology were detected in monkeys receiving the clinical blend of globulin either subcutaneously or intravenously.