Purified rat red blood cells, which express the A region antigens of the major histocompatibility complex (MHC) with which anti-RT-1 (H-1; Ag-B) haemagglutinins combine, fail to stimulate the production of either haemagglutinating antibodies or cellular immunity in allogeneic hosts. Allogeneic red blood cells may in appropriate circumstances be mildly immunosuppressive. Intraperitoneally injected allogeneic red blood cells persist in the host circulation for several weeks. The immune response to skin allografts does not accelerate the destruction of circulating donor strain red blood cells, although the administration of alloantiserum with a high haemagglutinin titre can do so. The manner in which allogeneic red blood cells effect immunosuppression is unknown. Approximately 75% of the red blood cells administered i.v. to nonimmune allogeneic hosts are destroyed, by mechanisms unknown, during the first 6 days after injection. By contrast, about one-half of this number of red blood cells are destroyed by syngeneic hosts during the same period. Red blood cells and platelets in rodents are probably similar in expressing A region (rats) or K-D region (mice) antigens of the MHC, in being nonimmunogenic by themselves and in their capacity for effecting limited immunosuppression. If human platelets have similar attributes, the administration of platelets bearing the appropriate HLA-A, B, and C antigens might be a useful adjunct to conventional immunosuppression in the management of allografts in man. Likewise, the platelet component of blood transfusions might account for their beneficial effect in some renal allograft recipients.