Investigating the functions of reticuloendothelial systems in the sensitized rabbits with hetero-organ antigens (brain, kidney, mucous membrane of small intestine etc.). Also the effects on sensitizations after administrations of Nitromin were observed. The following results were obtained. 1. Productions of antibodies for hetero-organ antigens were depressed in the sensitized rabbits with hetero-organ antigens blocking reticuloendothelial systems with Indian-ink. 2. The antibodies for hetero-organ antigens were slightly increased in the sensitized rabbits with hetero-organ antigens about 4 hours after the administration of Communin. 3. After the administration of Nitromin, the productions of antibodies for hetero-organ antigens were depressed in the sensitized rabbits with hetero-organ antigens. 4. Each antibody titers showed the highest for the corresponding hetero-organ antigens in the rabbits after the administration of Nitromin. 5. In the rabbits sensitized with hen's egg albumin instead of organ antigens, the antibody titers for the hen's egg albumin increased markedly. However, in the rabbits sensitized only for a short time, the cross reactions between the sera of the sensitized rabbits and the various hetero-organic antigens were hardly observed.