OBJECTIVE To investigate the optimal time for second allogeneic peripheral blood stem cell grafts (PBSC) harvest from healthy donors after in vivo recombinant human granulocyte colony-stimulating factor application (rhG-CSF). METHODS Thirty-eight healthy donors of second collection (group A) were treated with subcutaneous rhG-CSF \[5 microgxkg(-1)xd(-1)\] for five consecutive days and followed by leukapheresis on day 5 and 6. The control group (group B) was thirty-eight healthy donors who had received a first PBSC collection previously. Group A was reclassified as group C (< or = 9 months) and group D (> 9 months) according to the 75% quantile of interim time between first and second collection. The quantities of lymphocytes of CD3(+), CD3(+)CD4(+), CD3(+)CD8(+), CD14(+), CD34(+) cells and CD3(+)CD4(-)CD8(-) T cells were determined by multi-color flow cytometry. RESULTS The median number of CD3(+)CD8(+) (25.51 x 10(8)) and CD34(+) cells (0.51 x 10(8)) in group A were significantly lower than that (31.55 x 10(8) and 0.70 x 10(8) respectively) in group B (P < 0.05), and so did the CD3(+)CD8(+) (23.42 x 10(8)) and CD34(+) cells (0.42 x 10(8)) in group C than that in group B (P < 0.05). There was no statistical difference in median numbers of T cell subsets, monocytes, and CD34(+) cells between group B and group D (P > 0.05). The cell ratios of CD4(+)/CD8(+), CD14(+)/CD3(+) and CD3(+)CD4(-)CD8(-) T/CD3(+) in PBSC in group A, group C, and group D were similar to that in group B (P > 0.05). Sperman analysis showed a positive correlation between the total CD34(+) cells in second collection and the interval time from first to second collection (r = 0.357, P = 0.028). CONCLUSIONS Nine months after the first collection maybe an optimal time for the second PBSC collection. For those who undergo second PBSC collection within 9 months, more circulation blood should be extracted to ensure enough immunological and hematopoietic compositions.