The direct demonstration by McMaster and Hudack (1) of the production of antibody in lymph nodes led to a long search for the cell in the lymph node which synthesized antibody. Early evidence by Harris, Grimm, Mertens, and Ehrich (2), Dougherty, Chase, and White (3), and Harris and Harris (4) that the lymphocyte was the cell of synthesis was followed by the studies of BjSrnehoe and Gormsen (5), and Fagraeus (6) indicating the plasma cell as the cell which produced antibody. The following years produced studies of the cellular source of antibody by at least six experimental approaches, each of which led some authors to conclude that the plasmacytic series of cells was involved, and others, the lymphocytic series (7).The culmination, in 1955, of this decade and a half of intensive research was the actual finding of antibody within plasma cells, by Coons and his colleagues (8-10), using the indirect fluorescent antibody technique. These cells were found in antibody-producing lymph nodes; antibody-containing plasma cells were soon also found in other situations, i.e., in sites of deposition of transferred lymph node cells, by Dixon and his colleagues (11-13), and in chambers containing antigen-stimulated lymph node cells, by several groups of workers (14-16). Also supporting this association was the observation of Nossal (17), in single cell preparations from active lymph nodes, that virtually all the cells which were identified as antibody-producing were plasma cells.More recently, however, a number of reported observations have again given direct evidence of a role of the lymphocyte in antibody formation. In single cell droplet studies which involved a more sensitive antibody assay than that of Nossal (17), Attardi, Cohn, Horibata, and Lennox (18) found lymphocytes among the antibody-forming cells, the frequency of antibody-producing cells among the lymphocytes found being roughly one-third that found among plasma cells. Balfour, Cooper, and Alpen (19) found, after a secondary injection of diphtheria toxoid, that of the cells in the regional lymph node