Dogs were given transfusions of cryopreserved autologous mononuclear blood leukocytes after 1200 roentgens (R) (midline dose) whole-body x- irradiation. Bone marrow repopulation was studied by means of histomorphological methods at days 9 and 10 after transfusion of an average of 3 X 10(9), 7 X 10(9), 13 X 10(9), and 31 X 10(9) cells. The return of marrow cellularity to normal values was related to the number of cells transfused. With low cell doses (3 X 10(9) and 7 X 10(9)), the marrow regeneration at 10 days was focal. There were groups of cells (colonies) showing either erythropoiesis, myelopoiesis, or megakaryocytopoiesis in the osteal niches of the trabecular bones. Frequently such niches were seen showing complete cellular recovery next to niches with complete aplasia. With higher cell doses, all niches showed hemopoietic regeneration, and the cellularity approached normal values. No hemopoietic regeneration was observed in those skeletal parts that do not show hemopoiesis, even under normal circumstances.