Dinitrophenylated (DNP) conjugates of Ficoll, hydroxyethyl starch, levan, dextran, type 3 pneumococcal capsular polysaccharide SIII, pectin, alginic and hyaluronic acid at various epitope densities were tested for their capacity to inhibit secondary IgG anti-DNP antibody responses using an in vivo transfer system. Some aminobenzylarsonate conjugates were examined similarly. Conjugates of the acidic polysaccharides were markedly more effective than those of uncharged polysaccharides. Conjugates with high epitope densities were particularly tolerogenic. DNP conjugates of Ficoll and hydroxyethyl starch elicited prolonged IgM responses over a wide dose range, and there are indications in the literature that conjugates of dextran do likewise. All the polysaccharide conjugates, labeled with radioiodine, persisted in the body for long periods (half-lives 8.5–63 days), and all were cleared rapidly from, but remained detectable in the blood for many days. Gross tissue distributions varied markedly from one conjugate to another, and autoradiography revealed unexpected differences in cellular localization. Despite substantial retention in the liver, some uncharged polysaccharides were localized only in parenchymal and not in Kupffer cells. In the spleen, only the acidic polysaccharides were predominantly localized in red pulp macrophages, and the neutral polysaccharides were detectable exclusively, and in the case of Ficoll, hydroxyethyl starch and some dextrans very intensely in macrophages of the marginal zone of the white pulp. It is suggested that retention of thymus-independent antigens in marginal-zone macrophages favors, whereas trapping in red-pulp macrophages diminishes immunogenicity, and that tolerogenic effectiveness depends upon a balance between tolerization of B cells by free antigen and stimulation by antigen presented by marginal-zone macrophages. The distinction between functionally different macrophages in the spleen is elaborated in the following report.