The aim of the present study is to evaluate the kinetics of CD34⁺ cells and investigate the potential modulation of CD44 and CD31 expression on CD34⁺ cells during continuous i.v. administration of G-CSF, thus to elucidate the possible mechanism of peripheral blood progenitor cell (PBPC) mobilization. Fifteen healthy donors were enrolled in this study. G-CSF (10 μg/kg/day) was administered for four consecutive days through continuous 24-h i.v. infusion. For measurement of complete blood counts, CD34⁺ cell levels and their expression of CD44 and CD31, PB sampling was performed immediately before the administration of G-CSF (steady-state) and after 4, 8, 24, 48, 72, 96, and 120 h of G-CSF administration. The percentage and absolute number of CD34⁺ cells significantly increased at day 3 (0.55 ± 0.09%, 51.12 ± 24.83 × 10³/ml) and day 4 (0.47 ± 0.09%, 46.66 ± 24.93 × 10³/ml), compared to the steady-state level (0.06 ± 0.09%, 2.03 ± 5.69 × 10³/ml). At day 3 to day 5 following the onset of G-CSF administration, a strong decrease of CD44 and CD31 expression was observed on mobilized CD34⁺ cells compared to controls: the relative fluorescence intensity of CD44 and CD31 was, respectively, 50%-70% and 40%-90% lower than that of controls. We conclude that continuous i.v. administration of G-CSF apparently results in more rapid mobilization of CD34⁺ cells, and downregulation of CD44 and CD31 on CD34⁺ cells is likely to be involved in the mobilization of PBPC after treatment with G-CSF.