BACKGROUND: The aim of the present study is to evaluate the modulation of CD44 and CD31 expression and apoptotic status on mobilized CD34+ cells during continuous intravenous (i.v.) administration of granulocyte-colony stimulating factor (G-CSF), elucidating the mechanism of peripheral blood progenitor cells (PBPC) mobilization in normal donors. METHODS: Fifteen healthy donors were enrolled in this study. G-CSF (10 microgram/kg/day) was administered for 4 consecutive days through continuous i.v. infusion. Measurement of CD34+ cell levels and their expression of CD44, CD31 and 7-aminoactinomycin D (7-AAD) was performed. PB sampling was drawn immediately before the administration of G-CSF (steady-state) and after 4, 8, 24, 48, 72, 96 and 120 hours of G- CSF administration. RESULTS: Although there were considerable variations among the healthy donors, the absolute number of CD34+ cells significantly increased at day 3 (51.12+/-24.83x103/mL) and day 4 (46.66+/-24.93x103/mL),compared to the steady-state level (2.03+/-5.69x103/mL). The expression of CD44 on CD34+ cells revealed a significant reduction from the steady-state level (579.59+/-133.69) after day 3 (281.02+/-105.15, P=0.0059) and day 4 (164.76+/-107.44, P=0.0002) of G-CSF administration. The expression of CD31 on CD34+ cells also significantly decreased from the steady-state level (145.86+/-30.52) after day 4 (34.47+/-38.87, P=0.0055) and day 5 (17.33+/-50.68, P=0.0134) of G-CSF administration. The proportions of apoptotic (7-AADdIm) CD34+ cells were also significantly decreased after day 3 of G-CSF administration. CONCLUSION: Down-regulation of CD44 and CD31 on CD34+ cells is likely to be involved in the mobilization of PBPC and G- CSF may acts as a survival factor for mobilized CD34+ cells by the suppression of apoptosis during continuous i.v. administration of G-CSF in normal donors.