In an attempt to establish an efficient method of collecting peripheral blood stem cells and to utilize them for allogeneic peripheral blood stem cell transplantation, the effect of a combined administration of recombinant murine interleukin-3 (IL3) and recombinant human granulocyte colony-stimulating factor (G-CSF) to mobilize bone marrow stem cells into the circulation was examined in C57BL/6 mice. Some appreciable numbers (796 +/- 112/ml) of CFU-GM were recovered 6 days after G-CSF administration (500 micrograms/kg per day), while by IL3 administration (100,000 units/kg per day), the CFU-GM count was much lower (61 +/- 9/ml) with a small peak at day 4. By a combined administration of IL3 (100,000 units/kg per day) and G-CSF (500 micrograms/kg per day), the CFU-GM count at the peak of day 5 was significantly augmented (1178 +/- 277/ml) as compared to that of G-CSF or IL3 alone (P < 0.05). The CFU-S counts at day 5 (168 +/- 12/ml) and at day 6 (172 +/- 27) were also significantly higher than those of IL3 alone (day 5, 30 +/- 15/ml; day 6, 20 +/- 10/ml) or G-CSF alone (day 5, 114 +/- 14/ml; day 6, 112 +/- 19/ml). Thus the combined administration of IL3 and G-CSF appears to be promising for high yield collection of peripheral blood stem cells.