Removal of recirculating lymphocytes from the lymph of calves by extracorporeal thoracic duct filtration results in a state of immunounresponsiveness. Accompanying this is histological evidence of lymphocyte depletion from the lymph organs and structures of the body. Lymph nodes, spleen, hemolymph nodes, Peyer's patches, and thymus appear depleted of the recirculating small lymphocyte. We find this closed model of lymphocyte depletion effective and free of seyere problems with infections and physiological alterations as experienced in mechanical removal, extracorporeal irradiation, and open thoracic duct fistulas.