The effect of basic fibroblast growth factor (bFGF) alone and in combination with other hematopoietic growth factors on the colony formation of K562 human leukemic cells was studied using soft agar colony assay. bFGF was found to have a weak colony-stimulating activity on K562 cells derived from the blastic crisis cells of human chronic myelogenous leukemia and to potentiate the K562 cell colony-stimulating activity of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) and stem cell factor (SCF) at a low concentration of below 1 ng/ml. These findings suggested that bFGF stimulates the growth of human leukemic cells directly in vivo alone and in synergy with other hematopoietic growth factors.