The effect of recombinant human growth factors: rhIL-6, rhGM-CSF and rhEPO on normal human hematopoietic progenitor cells was studied by using liquid and semisolid culture systems. The results showed that the expanding folds of progenitor cells after culturing with the three growth factors were 4.3 +/- 0.6, 9.4 +/- 0.9 and 13.7 +/- 1.0 respectively, being markedly higher than those of control (2.4 +/- 0.7, P < 0.01. In the presence of IL-6 alone, no CFU-E colony was observed and only CFU-GM colonies were formed in the semisolid culture. When IL-6 was added with EPO, the number of CFU-E colonies was significantly higher than that of EPO alone (P < 0.01). In the presence of IL-6 plus GM-CSF or IL-6 plus GM-CSF and EPO, CFU-Mix colonies were observed. Moreover, the total number of colonies was significantly higher than that of IL-6 or GM-CSF alone (P < 0.01). It is suggested that IL-6 is an important hematopoietic regulator. IL-6 may act on hematopoietic progenitor cells to enhance their proliferation. The target cells of IL-6 are the same as those of GM-CSF. IL-6 has synergistic action with EPO and GM-CSF, to enhance erythropoiesis with EPO and proliferation of multipotential progenitor cells with rhGM-CSF and EPO. However, the mechanism of the synergism between IL-6 and other growth factors is still unknown.