For a better understanding of the spleen-dependent induction of the humoral immune response against thymus-independent type 2 antigens, we have studied the in vitro and in vivo localization of different capsular pneumococcal polysaccharides (PPSs) in the rat spleen. In this study, we found that in vitro binding of PPS types 3, 4, 6B, 9N/V, 14, and 23F was dependent on complement (probably a C3 fragment) and that the localization was predominantly restricted to the marginal-zone B lymphocytes and the follicular dendritic cells. In vivo, we observed with increase of time a shift of localized antigens. Shortly after injection, all PPS types localized in the marginal-zone B lymphocytes, then localized in the outer follicular mantle, and finally were found to be diffuse in the complete follicle and follicle corona. PPS types 3 and 9N/V and later also PPS type 23F localized additionally in red pulp macrophages. In particular, the localization in the marginal zone is important since the low flow in this area in combination with strongly CD21+ B cells, which are activated early, gives a maximum opportunity for the induction of a primary humoral immune response with subsequent differentiation into plasma cells or migration to the germinal center. In addition, the localization of PPSs at follicular dendritic cells should be considered important in the induction of an ongoing immune response not restricted to the spleen.