Hemopoietic recovery was studied in dogs given 2400 R fractionated total body irradiation within one week and graded doses of cryopreserved autologous bone marrow. Complete hemopoietic recovery including histology was observed after this dose and sufficient doses of marrow cells. Doses of more than 5.5 X 10(7) mononuclear marrow cells/kg body weight were sufficient for complete recovery in all dogs, 1.5 to 5.5 X 10(7) cells/kg were effective in some of the dogs and less than 1.5 X 10(7) cells/kg were insufficient for complete recovery. Similarly, more than 30,000 CFUc/kg body weight were required for hemopoietic recovery. The optimal marrow cells dose which has been defined as the minimal dose required for the earliest possible recovery of leukocyte and platelet counts was 7-8 X 10(7) mononuclear marrow cells/kg body weight. It has been concluded that fractionated total body irradiation with 2400 R does not require greater doses of marrow cells for hemopoietic reconstitution than lower single doses and that the hemopoietic microenvironment is not persistently disturbed after this dose.