A study was made of the cellular and humoral aspects of the immune response of the rabbit to the somatic polysaccharide of Salmonella enteritidis. The response to a single intravenous injection was characterized by the appearance of elevated titers of bactericidal antibody between 2 and 3 days later. The maximum titer was dose-dependent and occurred between 5 and 7 days, thereafter declining rapidly during the first month. The significant stabilized levels which then persisted for at least 1 year were also dose-dependent. Most of the antibody produced (>99 per cent) was associated with the macroglobulin fraction of serum. Plaque-forming cells (PFC) elaborating antibody specific for this somatic antigen were detected and enumerated by the technique of localized hemolysis in gel employing polysaccharide-coated sheep erythrocytes. Significant numbers of PFC were encountered in the spleen as early as 14 to 18 hours after a single intravenous injection of antigen; after 36 hours the number of PFC rose rapidly and culminated in a maximum population at 5 days, followed by a rapid decline and plateau similar to that for circulating antibody. The spleen was the principal organ involved in the systemic response, but other lymphoid tissues including bone marrow, peripheral blood leucocytes, and thymus contributed significantly. After an interval of 3 months the effect on humoral antibody titers of a second injection of antigen was dependent on the amount of polysaccharide administered; markedly greater titers were now obtained with 0.02 to 0.002 µg, whereas 0.2 to 20 µg resulted in a duplication of the initial humoral response. The cellular response to a second dose of 5 µg was accelerated; larger numbers of PFC appeared more rapidly, attained a maximum population by day 3, and exceeded the primary response by a factor of two. This acceleration in the attainment of maximum numbers of PFC and the increased bactericidal antibody titers following a second injection of limiting amounts of antigen suggest that these somatic polysaccharides may in fact evoke a "secondary" type of response in the rabbit.