Thymosin, a lymphocytopoietic preparation obtained from calf thymus, was examined for its effect on the rate of regeneration of lymphoid tissue in CBA/W mice exposed to whole body x-irradiation. In CBA/W mice receiving either an acute (400 R) or a lethal (700 R) dose of whole-body x-irradiation, thymosin was found to have a potent stimulatory influence on lymphoid tissue regeneration. Administration of thymosin daily, beginning at 1 1/2 hours after irradiation of the animals, significantly accelerated the rate of weight increase and3 H-thymidine or${}^{3}{\rm H}\text{-deoxycytidine}$ incorporation by spleen and pooled brachial, axillary, and inguinal lymph nodes. Animals treated with thymosin daily for 7 days prior to being subjected to x-irradiation had, initially, larger lymphoid structures which were less severely involuted by radiation and exhibited an accelerated rate of regeneration subsequent to exposure to x-rays. Radio-autographic sections of the lymph nodes and spleens of the thymosin-treated groups demonstrated that thymosin was acting primarily upon the more primitive lymphoid elements. Incorporated3 H-thymidine was localized in the immature lymphocytes; only rarely did small lymphocytes display radioactivity.

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