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The effect of thymectomy at different ages in C3H mice on development of circulating leukocytes and cells of marrow, spleen and lymph nodes has been analyzed. Regardless of the age at which thymectomy is performed depression of numbers of circulating lymphocytes is produced. Tliymectomy at birth did not affect significantly the relative number of lymphocytes in the marrow during the first few weeks of life, later they fell to low levels. Thymectomy a t four weeks was followed by prompt reduction in relative numbers of lymphocytes in the marrow.After reaching six weeks of age, neonatally thymectomized mice showed a high proportion of monocytes in the marrow. Neonatal thymectomy and thymectomy at two weeks of age reduced the number of eosinophils in the marrow.Neonatal thymectomy inhibited development of lymphocytes in the spleen, whereas thymectomy later in life produced only transient depression of lymphocytes in this organ. In addition, neontal thymectomy decreased the relative numbers of small lymphocytes in the lymph nodes. This was associated with drastic depletion of lymphocytes in the deep cortical regions of the nodes.It has now been well established that thymectomy in the neonatal period in rodents, exerts a profound effect on development of lymphoid tissue and immunologic capacity (Archer and Pierce,

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The effects of splenectomy at birth and two and four weeks of age on the development of lymphoid and hematopoietic tissues has been compared in C3H/Bi mice. During the neonatal period normal mice showed changes in total leukocyte count, and in absolute members of the specific cell types. The predominant changes included an initial fall of numbers of granulocytes and steady increase in numbers of lymphocytes during the first six weeks of life. Neonatal splenectomy interfered with the normal developmental pattern by interfering with normal growth of numbers of circulating lymphocytes while splenectomy at two and four weeks of age produced increases in numbers of both circulating lymphocytes and granulocytes. Neonatal splenectomy produced effects on development of bone marrow, peripheral lymph nodes and thymus. The major influence seemed to involve decrease in percentages o f small, densely stained lymphocytes in thymus and peripheral lymph nodes.