This study compares the pattern of haemopoietic recovery in dogs after total-body irradiation and transfusion of different populations of cryopreserved autologous bone marrow cells. Dogs in group 1 received unseparated marrow cells. Group-II dogs were transfused with small (less than 5.1 mm/h) and group-III dogs with large (greater than 7.1 mm/h) bone marrow cells, separated by velocity sedimentation. Myeloid progenitor cells (CFU-GM) present in slowly sedimenting cell fractions were characterized by a higher radiosensitivity in vitro and a lower proportion of cells in S-phase as compared to rapidly sedimenting CFU-GM. Although autografts in all groups contained comparable numbers of CFU-GM, transfusions resulted in different patterns of recovery. Fractions of small bone marrow cells contained most of the pluripotent stem cells, leading to speedy haematological reconstitution and long-term survival. The pattern of early recovery was similar in dogs of group I and of group II. In group III, the recovery in all cell lineages was delayed, going along with marked extramedullary haemopoiesis. The data may indicate the limits of committed progenitors in reflecting pluripotential stem cells, in particular, if bone marrow grafts have been modified in vitro. Differences in the repopulating potential of the graft might be reflected by distinct physical and biological properties of the CFU-GM.