The effect of G-CSF, GM-CSF, IL-1 and IL-3 on the proliferation of acute leukemia cells (evaluated as (3)HTdR uptake) was investigated in short-term liquid cultures and compared with that observed on normal bone marrow (BM) populations enriched for immature cells. In acute myeloid leukemias (AML), a marked leukemic proliferation was induced in 10/18 cases by IL-3, in 9/18 by GM-CSF, in 7/18 by IL-1 and in 4/18 by G-CSF. In acute lymphoid leukemias (ALL), marked stimulation was observed in 7/11 cases with IL-3 and in 5/11 with GM-CSF, whereas IL-1 and G-CSF were ineffective. Both in AML and ALL, the combination of several factors did not result in an additive synergistic effect. Purified normal BM cells responded to all four growth factors and their combinations produced an additive effect on cell proliferation which probably relates to the heterogeneity of the cell populations studied. The effect of a G-CSF, GM-CSF, IL-1 and IL-3 on the proliferation of acute leukemia cells by different growth factors suggests that caution should be exercised in their clinical use in these diseases.