Mice received G-CSF and SCF subcutaneously each day from day 17 to 20 of parabiosis. To quantitate blood HSCs in studies at 6 weeks (d 42) with cytokines at days 17 to 20, blood cells from 2-ROSA or 2 PeP3^b parabionts were pooled before secondary transplantation, and no endogenous reconstitution was seen in the secondary BL6 host. Pooling of blood samples was not required for studies at 3 weeks (1 day after cytokine exposure). As seen, 1 day after cytokine exposure, the percentage of partner HSCs in marrow had not increased substantially from baseline values, suggesting that HSCs that exited marrow had not yet returned or that replication of resident HSCs (self-renewal) increased. In contrast, many (10.1%) of marrow HSCs were of partner origin 3 weeks later. At this time, 10.6% ± 5.5% of marrow CFU-GMs and 10.9% ± 6.2% of marrow granulocytes were also of partner origin. NR indicates no reconstitution of irradiated recipients.