1) Removal of the tymus from guinea pigs within 24 hours after birth causes decrease in number of immunologically competent cells in the tissues, in ability of serum antibody production and in manifestation of delayed hypersensitivity of the skin 2) The thymus cells of guinea pigs are cytomorphollogically divided into large and small lymphocytes, lymphoreticular cells, lyphoblasts and mesenchymal reticulum cells. In addition, there are epithelial reticulum cells, tissue mast cells, neutrohils and plasma cells 3) It is assumed that the lymphoreticular cells are the most possible immunologically competent cells in the thymus. These cells imigrate out of the border portion between the cortex and medulla to the medulla of the thymus through the lymph vessels that lie between the cortex and medulla and settle down in the remote lymphatic tissues. Lymphoreticular cells are also considered as the most imporant immunologically competent cells in these remote lymphatic tissues.