The effects of human recombinant granulocyte and granulocyte-macrophage- (G- and GM-CSF), and of purified macrophage-stimulating factors (CSF-1), were tested on populations of leukemia cells isolated from 18 patients with different types of acute myeloid leukemia. Cell proliferation and differentiation were studied by culturing the cells in suspension for 7 days in the presence of CSF or medium alone. Spontaneous cell proliferation, as assessed by tritiated thymidine uptake, was observed in 9 of the 18 cases. GM-CSF induced proliferation in seven of the nine cases without spontaneous growth and increased spontaneous proliferation in nine cases. G-CSF added alone was also found to strongly stimulate leukemic blast cell proliferation, in which a translocation involving the long arm of chromosome 17 was observed. Low levels of CSF-1 stimulation were also observed in some cases. No clear morphological modification supporting evidence of terminal differentiation was observed, whereas modulation of some cell surface antigens was detected by flow cytometry. Thus, most leukemia cells still depend on growth factors for their proliferation, GM-CSF appearing the most effective. On the other hand these factors were not able to induce terminal differentiation.