We investigated the effects of low-dose granulocyte colony-stimulating factor (G-CSF) on the mobilization of stem cells in 6 healthy subjects. When G-CSF was administered by continuous subcutaneous infusion at a rate of 72 microg/day for 5 days, the numbers of white blood cells and granulocytes rapidly increased to maximal levels. CD34+ cells were mobilized to the peripheral blood in 3 days, and the maximal level was reached 4 or 5 days after the start of treatment. We attempted to determine whether the levels of mobilized stem cells that we could obtain using this method would be sufficient for peripheral blood stem cell transplantation. Two of the 6 subjects complained of mild bone pain 4 or 5 days after the start of treatment, but the pain did not affect their daily activities. Only 1 abnormal result (for serum alkaline phosphatase) was found in the laboratory data. The present preliminary results have provided us with a framework for a prospective study comparing low-dose continuous infusion with conventional mobilization procedures.