Sublethally irradiated AB/Bln, mice (800 c Gy) and supplemented with syngeneic bone marrow cells were treated with or without a splenopentin derivative (DAc SP-5) and compared for their capacity to produce antibodies. In bone marrow cell plus DAc SP-5 treated animals antibody forming cells were found earlier and in a higher frequency than in mice treated with bone marrow cells only. These findings demonstrate that DAc SP-5 is able to induce bone marrow cell maturation.