Many investigators have recorded experiments designed to show the participation of one cell or another in antibody formation, and the experimental evidence for each has recently been reviewed by McMaster (1) and by Good (2). However, due to the sparcity of cellular reaction incited by a single injection of antigen, a lack of cytologic description and information exists concerning the origin and mechanism of differentiation of antibody forming cells following an initial a~tigenic stimulus.In this regard, McNeil (3, 4) has examined the changes in proximal lymph nodes of rabbits induced by single and multiple subcutaneous injections of bovine serum albumin. Primary stimuli apparently caused a hyperplasia of lymphobiasts, lymphocytes, and plasma cells, while multiple stimuli resulted in the appearance of giant cells. The latter were postulated by McNeil to be due to a stimulation of reticular cell elements of the medullary portion of the lymph nodes. Using the same antigen, Makinodan and Wolfe (5, 6), examined the spleens of chickens at various intervals after a single injection, and considered the cells important in antibody formation to be derived from the plasma cell lineage. Blast forms predominated 2 days after the administration of the antigen, with definitive plasma cells appearing on the 4th day, 2 days prior to the presence of circulating antibody.Coons and associates, utilizing a fluorescein labelling technique (7-10), examined both the primary and the hyperimmune response in several rabbit tissues. After the subcutaneous introduction of soluble diphtheria toxoid, changes were observed primarily in the medullary areas of regional lymph nodes. Differentiation toward plasma cells was observed. A second stimulus increased markedly the number of cells containing demonstrable intracellular antibody. In the spleens of hyperimmune rabbits