Treatment of mice with high doses of endotoxin (ET) affected the CFU(s) population in bone marrow, spleen and liver. By 7 days after injection the number of CFU(s) in bone marrow had decreased and the number in spleen and liver had increased. The experimental data show that if mice were irradiated and reconstituted with bone marrow 7 days after ET treatment, the lodging pattern of injected cells followed qualitatively the same changes as the changes in numbers of CFU(s) present before irradiation in these organs. However, elevated numbers of CFU(s) cannot be maintained in the liver and can only partly be maintained in the spleen. In retransplantation experiments, CFU(s) from bone marrow and from bone marrow or liver after treatment of mice with ET have the same seeding efficiency in the spleen, indicating that there is no preferential homing of subclasses of CFU(s) in bone marrow or liver. It is concluded that the presence of CFU(s) and hemopoietic tissue may be an expression of the capability of the tissues for the maintenance of lodging CFU, but the initial lodging in these organs is also affected by other trapping mechanisms.