C57BL/6 mice received whole-body x-irradiation, and differential cell counts were made on cellular suspensions of the lymphatic tissues at regular intervals. A two-phase pattern of thymic lymphopoiesis was demonstrated: The first phase of blast cell proliferation lasted for three to four generations and was followed by a period of differentiation to small lymphocytes. The difference in the time of repopulation between the thymus and the lymph nodes was ascribed to the difference in the absolute number and the doubling time of primitive cells. The doubling time of blast cells was 12 hours in thymus and 20 hours in lymph nodes. The relation between dose and the number of surviving cells at 24 hours after irradiation revealed the existence of two classes of lymphocytes with different radiosensitivity. The <tex-math>$D_{37}$</tex-math> was 145 R and 425 R for small lymphocytes and 57 R and 520 R for medium and large lymphocytes in thymus. The <tex-math>$D_{37}$</tex-math> values for small and larger lymphocytes in lymph nodes were 213 R and 200 R, respectively.

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