In the chicken spleen there are two distinctly different types of lymphoid tissue. One is seen along the small arteries and arterioles, as shcaths of small lymphocytes or as scattered clusters conposed of large and small lymphocytes, lymphoblasts, primitive reticular cells, and frequent mitotic figures, This represents the white pulp of the spleen, the bulk of its lymphoid tissue, and it becomes apparent in normal chickens in the immediate post-hatching period. We found that it was markedly reduced in thymectomized-irradiated and thynaectornizedburscctomized-irradiated chickens. Such depletion has been noted previously in chickens subjected to neonatal surgical thymectomy" and in birds with testosteroneinduced thymic cortical damage?. This component was not significantly altered in our other experimental groups. This lymphoid tissue we consider the thynus-dependent lymphoid tissue of the chicken. There is a second type of lymphoid tissue in the chicken spleen, clearly differentiable from the thyrıus-dependent type both in morphologic appcnranco and in tho time of its development. It is seen as sharply circumscribed round or oval lynphoid follicles (Fig. 1) which seem to be encased by a thin fibrous mcmbranc and which always lie in juxtaposition to a small artery. These follicles bear a striking morphological resemblance to the follicles of the bursa of Fabricius itself (Fig. 2). They are even more clearly separable from the other spleen components when