FLT-1 neutralization impedes the exit of ALL cells from the BM to the peripheral circulation. As described in “Materials and methods,” total mononuclear cells were isolated from the peripheral blood (PB) of inoculated NOD-SCID mice, and RNA was extracted to allow screening for human cells (as determined by human CD19 expression) in the circulation by RT-PCR. Results show that at day 19 after leukemia-cell inoculation, it was possible to detect circulating blasts in untreated mice and those treated with the KDR Ab. Conversely, mice treated with the neutralizing FLT-1 Ab had no evidence for circulating leukemia cells at this same time point. These data are representative of 3 independent experiments, with the same results.