In a preceding report (1) we described a method which permitted detailed investigation of the fine structure of antibody-producing ceils obtained from lymph nodes. These cells, from rabbits injected with sheep erythrocytes, were recognized by the hemolytic plaques which they produced on incubation with the target erythrocytes in vitro. Such plaques were removed for study of the central cell by electron microscopy. We found that cells in the category both of lymphocytes and of plasma cells could produce antibodies of the 19S type after a single injection of the antigen. The lymphocytic cells showed an increase in structures presumably associated with synthetic functions, such as endoplasmie reticulum, Golgi body, and nueleolus, not present in the quiescent small lymphocyte. The gradations of organelle development through a series of cells which included lymphocytic and plasmacytic forms suggested that in the lymph node the antibody-producing cells underwent a progression of development leading from the small lymphocyte to the mature plasma cell, with a great variety of intermediate stages. Of these cell types some, of the plasmacyfic series, have been identified in other electron microscopic studies of antibody-forming lymph node cells by Bussard and Binet (2), and by Fitch, Rowley, and Coulthard (3).The antibody-producing cells in our earlier study (1) were obtained from the lymph node by teasing the excised organ, and thus might not represent the ceils which under physiological conditions would pass into the efferent lymph and eventually into the peripheral blood. These extranodal antibody-producing cells could represent the most highly organized types, with respect to their fine structure, since they were presumably fully developed in the lymph node before leaving it to enter the efferent lymph vessel. Such cells, obtained from efferent lymph, the thoracic duct, and peripheral blood were investigated and the results are presented.

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