Recent advances in molecular biology have led to the identification of hematopoietic growth factors that support and influence the proliferation of hematopoietic progenitor cells in vitro and in vivo. Although these factors have been extensively studied, little is known of their role in the regulation of cell-cycling of hematopoietic progenitors, especially in the early stage of hematopoiesis. In the present study, we examined the effects of early acting growth factors on proliferative kinetics of hematopoietic progenitors by monitoring the number of cells in individual developing colonies, using an in vitro clonal assay. Interleukin-11 (IL-11) or steel factor (SF), alone or in combination, shortened the time for the size of IL-3-dependent colonies to double. Consecutive replating experiments provided evidence for direct action of growth factors on the growth rate of hematopoietic progenitor cells. Shortening of the time for the total cell number in the colonies to double was due to a reduction in time for each single cell within the respective colonies to become two daughter cells, and there was no alteration in the incidence of cells with a proliferative capacity. Cell-cycle analysis demonstrated that IL-11 has the potential to induce a shortened time for cell-cycle of hematopoietic progenitor cells without affecting distribution of each fraction of the cell-cycle, whereas SF has the potential to reduce cell-cycle time mainly by decreasing the time required for hematopoietic progenitor cells to go through the G1 phase. These results suggest that growth factors may modulate cell-cycling of hematopoietic progenitor cells.