AbstractLymphopoiesis with respect to recirculating and non‐recirculating small lymphocytes was measured simultaneously in rats thymectomized as adults. Removal of the thymus at four to five weeks of age had a profound inhibitory effect upon the production of recirculating cells, whereas the formation of non‐recirculating lymphocytes was only slightly depressed. Thymectomy had approximately the same impact of lymphopoiesis as thymectomy and exposure of the animal to a large dose of whole body X‐ and γ‐irradiation. The latter finding, and the failure of a thoracic duct cell transfusion to augment lymphocyte production, accord with the view that the thymus is the principle intermediate source of recirculating small lymphocytes in the normal, unstimulate
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