Rat bone marrow is deficient in immunologically competent cells when compared with the peripheral pool of circulating small lymphocytes. However, circulating lymphocytes capable of initiating a lethal graft vs. host reaction can develop from myeloid precursors. In adult rats inoculated at birth with parental strain bone marrow cells, the thoracic duct lymph contains cells having the immunological capability of the marrow donor. The evidence suggests that immunologically competent lymphocytes derive from cells normally resident in bone marrow and probably not from precursors in the peripheral lymphocyte pool.

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