Lewis rats show prolongation of survival of LBNF1 renal allografts when those grafts are drained by the portal vein, or if recipients are treated with LBNF1 bone marrow cells infused via the portal venous route peritransplantation. The longest survival was seen in animals in which both portal venous transfusion and graft drainage by the portal route were performed. When the same manipulations were performed for Lewis rats receiving heterotopic small bowel transplants, only in the "combined treatment" group was there significantly enhanced graft or animal survival relative to control rats. In separate studies, we examined the mixed leukocyte proliferation response in vitro, and IL-2 production, from rats treated as above and receiving renal or small bowel transplants. In both organ transplant models, there was a good correlation between enhanced graft survival in vivo and decreased in vitro responses to specific allostimulation.