The effect of immunosera (rabbit) against the lymphoid cells of mice sensitized with antigens causing development of hypersensitivity of delayed type (tuberculosis bacillus, tissue antigen from rabbit kidney) on the specific activity of lymphoid cells of guinea pigs was tested. The mentioned sera blocked the capacity of lymphoid cells of guinea pigs to transfer the state of sensitization from the sensitized donors to the intact recipients. Blocking was strictly specific and this capacity was absent in the immunosera against the normal lymphoid cells.