Guinea pigs may be safely desensitized and made immunologically tolerant following immunization with Type III pneumococcal polysaccharide. This can be done safely by inserting a preparation of the polysaccharide that has been made insoluble by cross linking the molecules of the polysaccharide. By contrast attempts to tolerize allergic guinea pigs with soluble Type III polysaccharide always resulted in death. Attempts to extend the studies to a hapten were made. Guinea pigs were immunized to hapten dimitrophenol. That was coupled to dextran and sephedex beads to make an insoluble product. Although the products were not entirely insoluble, they too proved to be a safe material with which to tolerize the guinea pigs.