Quantitative myelograms, peripheral blood counts and the histologic appearance of lymphoid tissues were studied in albino mice of the CFW strain during a period of 4 weeks following single whole body radiation of medium dosage.

Nucleated red blood cells disappeared almost completely from the marrow within 24 hours after irradiation. Regeneration of the erythroid series commenced around the 7th day. Only mild to moderate anemia developed, presumably due to the longevity of red blood cells.

Suppression of mitotic activity and diminution of the myeloid marrow was marked during the first week after irradiation. Granulocytopenia in the peripheral blood was severe for a very short period only. In contrast, early damage to the lymphoid tissue was more pronounced and lymphopenia was of longer duration than granulocytopenia in spite of early regeneration of the lymphoid tissues. Differences in the relative amount of damage to lymphoid and myeloid tissues and in their rates of regeneration were not of sufficient magnitude to indicate a truly selective damage to the lymphoid tissues under the conditions of our expermiments.

No morphologic evidence of damage to the reticuloendothelial system was found. The possible use of irradiated animals for functional studies of the lymphoid apparatus is discussed.

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