The optimal time interval between termination of radiation therapy and marrow grafting has been studied in an animal model of syngeneic bone marrow transplantation, using a limiting number of bone marrow cells. Optimal survival was achieved when reconstitution took place 24 h following termination of radiation therapy at all cell doses studied, including 5 x 10 (4), 2 x 10 (5) and 1 x 10 (6) cells/recipient. The worst results were observed when bone marrow cells were infused immediately after irradiation. These results suggest that reconstitution of lethally irradiated recipients requires migration of irradiated host marrow cells and establishment of adequate'bone marrow space', which is optimal at 24 h following termination of whole body irradiation.