Modification of the immune response towards the generation of suppressor T cells can be attained using experimental designs promoting immunological feedback mechanisms. Those studies have been carried out in defined inbred animal systems. With respect to the great importance for organ transplantation, an in vivo model using larger outbred animals was developed in order to study such feedback reactions. The following experimental design was used: (1) allogeneic spleen transplantation in the dog; (2) autologous retransplantation of this spleen together with an allogeneic kidney transplantation to the former donor of the spleen after a period of 2-4 days. Although full immunological activity of the transplanted as well as the retransplanted spleen was demonstrated, rejection of the kidney occurred in a fashion identical to that observed in control animals.