We have studied the effects of medium conditioned by the human progranulocytic leukemia cell line, HL-60, on the subsequent growth of new inocula of HL-60 cells. When HL-60 cells were cultured at high cell density, optimal growth rate occurred in liquid suspension and confluent colony growth was observed in viscous medium without the addition of conditioned medium. However, when cells were cultured at lower cell density, growth rate was reduced and colony growth was nil unless conditioned medium from HL-60 culture was added. All HL-60 populations studied, including the earliest available passage, 9, both elaborated and responded to HL-60 CM. HL-60 CM did not stimulate normal human or mouse granulocyte-monocyte colony-forming cell (CFU-GM) growth. Conditioned media from other human cell lines varied in the ability to stimulate HL-60 cell and CFU-GM proliferation. Some, such as GCT CM, stimulated both HL-60 cells and normal CFU-GM, whereas others, like HL-60 CM, stimulated only HL-60 growth. The majority of cell line CMs tested did not stimulate either HL-60 or CFU-GM. Chromatography of HL-60 CM on Ultrogel AcA54 showed a single peak of HL-60 stimulating activity of apparent molecular weight 13,000. The ability of HL-60 cells to elaborate this activity provides a possible explanation for their proliferation at higher cell densities. Autostimulation may prove to be important in the high growth potential of other cell populations that undergo unrepressed proliferation.