Four immunization routes were investigated to induce an immune response against three structurally different types of pneumoccoccal polysaccharide (PPS) in the rat. In particular, the contribution of the IgA isotype in these immune responses was studied. Six days after administration of PPS type 3, 4 or 14, the localization of specific antibody-containing cells (ACC) in different lymphoid tissues and the antibody titres in serum were studied. All four routes induced anti-PPS ACC in the spleen. After intraduodenal, intravenous and especially intraperitoneal administration of PPS, many IgA-specific anti-PPS ACC were also found in parathymic and mesenteric lymph nodes and in the lamina propria of intestinal tissue. Several anti-PPS ACC were found in Peyer's patches, located peripheral of the B-cell areas. The intratracheal immunization elicited only a local immune response, in bronchus-associated lymphoid tissues and paratracheal lymph nodes. The localization of these anti-PPS ACC was influenced by the route of immunization. After all four investigated routes, specific antibodies were found in serum against PPS. However, some remarkable differences between PPS-3, 4 and 14 were found in the magnitude of the immune response and the distribution of the isotypes. Both route of immunization and structure of the PPS have a profound influence on the immune responses in rats.