[en] Experiments were carried out to specify the adrenaline target among the immunocompetent cells. Adrenaline administered for some hours exerted opposite effects on the natural PFC and RFC: the first were enhanced and the second significantly reduced. These paradoxical results were interpreted as a consequence of the inhibition of the suppressor T-cells in the resting status. Adrenaline appeared to act on the sensitive cells through beta- rather than through alpha-receptors. Further experiments on the adrenaline influence on the syngeneic barrier phenomenon and on the cellular balance at its termination seemed to indicate that adrenaline was directly inhibitory for the Ts but not for their precursors. These results are discussed in the light of the cellular networks regulating the immune response. Irradiated mice were compared with non-irradiated mice as described in the previous article. (Auth.)