This study aimed to correlate blood Flt3-ligand (FL) concentration with CD34(+) cell number in blood and bone marrow (BM) during granulocyte colony-stimulating factor (G-CSF) mobilization. Nonhuman primates were injected with 10 microg/kg of G-CSF (Lenograstim) daily over a period of 5 days. Daily blood sampling and repeated BM sampling showed that FL concentration before mobilization was negatively correlated to the absolute number of BM CD34(+) cells, but also to the number of G-CSF-mobilized CD34(+) cells on days 3-5 of treatment. This showed that FL concentration in the blood reflected BM status before mobilization, and suggested that this parameter could be used as a predictive indicator of G-CSF-induced CD34(+) cell mobilization.