T- and B-populations of lymphocytes from the thymus, spleen and mesenteric lymph nodes have been studied. Rabbits of various age were immunized intravenously with suspension of 20 . 10 g/kg sheep erythrocytes. The results of the histochemical and scanning electron microscopic investigation demonstrate that in 6-month-old animals B-lymphocytes transform intensively into plasmic cells. Under antigenic stimulation, certain changes of the lymphatic surface are demonstrated. Enzymatic activity specific for T- and B-depending zones is stated.