In mice treated with phenylhydrazine haemopoietic spleen colony forming cells (CFU-S) are proliferating rapidly in the bone marrow but not in the spleen. Using such mice we have investigated the production of factors responsible for the control of CFU-S proliferation. When irradiated spleen cells are incubated with non-irradiated bone marrow cells there is a marked fall in the proportion of femoral CFU-S in DNA synthesis. In the converse experiments, rapid triggering of splenic CFU-S is achieved. Both these effects can be eliminated by washing the irradiated cells prior to incubation; they are, however, retained in the supernatant media "conditioned" by these cells. When the washed cells are incubated in fresh medium at 37 degrees C both stimulatory and inhibitory activities reappear but after different incubation periods. The data demonstrate that both proliferation stimulatory and inhibitory factors acting on CFU-S can be produced by the same haemopoietic cell suspension.