The recent development of two continuously proliferating human myeloid leukemic cell lines (HL-60 and KG-1) that response to CSA provides an opportunity for a detailed study of the interaction of CSA with leukemic myeloid cells. Here we report on the colony-forming ability of HL-60 and KG-1 over an extended culture life of the cells. Several different sources of human CSA of different stages of purity enhanced colony formation of these cells. CSA, obtained from conditioned media from an SV-40 transformed human trophoblast, was partially purified, and its activity for normal bone marrow copurified with the activity that stimulated HL-60 colony formation. Over 100 clones of HL-60 were developed and tested for their response to CSA. All responded to CSA by showing an increase in colony size and number. However, none of the colonies formed from any of the 100 clones differentiated in response to CSA despite the fact that many chemical can induce differentiation of HL-60. since HL-60 forms spontaneous colonies without the addition of any exogenous stimulating factors, HL-60 conditioned media and cell extracts were tested for the production by these cells of their own endogenous growth-promoting activity (such as a CSA-like molecule). No growth-promoting endogenous activity was found that stimulated normal bone marrow or HL-60 colony formation even after concentration and fractionation methods were employed. These experiments suggest that: (1) the effect of CSA markedly favors proliferation over differentiation in these cell lines; (2) CSA is unlikely to suppress growth of the age of the type of leukemic myeloid cells that HL-60 and KG-1 represent; and (3) if HL-60 cells produce their own growth-promoting factor it is not detectable in the media.