The purpose of this experiment was to compare the pattern of hemopoietic regeneration following bone marrow aplasia in conventional and germfree${\rm ND}_{2}$ mice after 700 rads whole body x-ray exposure. All conventional mice died of the consequences of pancytopenia, i.e., bleeding and infection, within 10-12 days postirradiation. In these animals, no sign of bone marrow restoration could be detected; however a limited amount of hemopoietic recovery was seen in the spleen. In contrast, about 40% of the germfree mice survived the first 12 days of postexposure. Quantitative cyto- and histomorphological studies of the blood, bone marrow and spleen provided evidence of a progressive hemopoietic recovery which could be followed in some of the animals for more than 20 days postirradiation. The first evidence of erythro- and megakaryocytopoietic regeneration in the bone marrow was observed at days 10 and 16, respectively, postirradiation. Blood erythrocyte and platelet counts began to rise between days 15 and 18. However, no signs of myelopoietic recovery were seen within the first 3 weeks postirradiation. Endogenous spleen colonies appeared between days 12 and 16 in the germfree mice. The nodules consisted mainly of erythropoietic tissue; however some were found to contain megakaryocytic and myelopoietic cells as well. Possible reasons for the differences in the response of germfree and conventional animals to irradiation are discussed.

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