Regeneration of the splenic tissue after partial splenectomy is incomplete in adult non-irradiated mice and lethally irradiated animals reconstituted with donor syngeneic bone marrow. Transplantation of the splenic tissue to intact adult animals after partial splenectomy resulted in virtually complete regeneration of the spleen. In chimeras recovery of the splenic tissue was decreased; autotransplantation of the whole spleen or its part did not lead to appreciable changes in the weight and cellularity of this organ. No more than 30% splenic tissue is restored after complete splenectomy and transplantation of the splenic tissue in intact and chimeric mice.