The biological mechanisms associated with the development of the primary immune response have occupied the attention of numerous investigators. Such fundamental immunological features as the induction phase (1-3), the duration of the latent period (4-6), the cellular dynamics associated with antibody formation (5-9), the homeostatic mechanisms controlling the course of the immune response (10-12), and the development of immunological memory (13-15) have all been extensively considered.The various parameters of the primary immune response have been characterized in the past largely in terms of the appearance of various kinds of circulating antibodies. Such studies may fail to delineate some aspects of the immune response which can only be derived by analyses of the immune reaction on a cellular level. The plaque technique of Jerne and coworkers (16), which demonstrates the production of 19S immunoglobulin to sheep erythrocytes, permits such an analysis. Moreover, the recent modification by Dresser and Wortis (17) for the detection of 7S immunoglobulin production to sheep erythrocytes now allows the assessment of the immune response with respect to both types of immunoglobulins.In the present work, some of the biological features of the primary immune response were studied on a cellular level in a group of Swiss white mice. Such factors as the sequential pattern in 19S and 7S immunoglobulin production with respect to time of onset and duration of immune response were assessed after intradermal (footpad) and intravenous administration of sheep erythrocytes. Several additional features, such as the characteristics of the 19S and 7S peaks, were also considered and compared with similar manifestations in the seconddary response. Comparison was also made of the relative contributions of the proximal and distal lymphoid tissues to the total immune response of the host.