The effects of recombinant human stem cell factor (SCF, a c-kit ligand) on an eosinophil lineage were examined in clonal and suspension cultures of human non-adherent light density bone marrow cells. Although interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-5 (IL-5) each exhibited eosinophil-colony stimulating activity, SCF did not do so alone. However, the addition of SCF to IL-3, GM-CSF, or IL-5 cultures led to an increase in the number of eosinophil colonies per 5 x 10(4) cells, from 8.0 +/- 1.4, 11.0 +/- 2.0, and 6.7 +/- 0.6, to 12.7 +/- 3.2, 19.0 +/- 4.4, and 12.0 +/- 2.0, respectively. A similar synergistic effect of SCF on eosinophils was also observed in the suspension cultures of bone marrow cells, although SCF alone had little proliferative effect. Moreover, although the delayed addition of IL-5 to cultures containing SCF led to a small increase in the number of mature eosinophils, the effect of SCF was less than that of either GM-CSF or IL-3. These observations suggest that SCF may have a proliferative effect on eosinophil precursor cells and may increase the number of mature eosinophils when used in combination with such other growth factors as IL-3, GM-CSF, and IL-5.