Purpose. Very late antigen-4 (VLA-4) and vascular cell adhesion molecule-1 (VCAM-1) are known to be involved in the bonding of hematopoietic stem cells (HSCs) to stromal cells in bone marrow (BM). Breakage of these bonds is essential to allow mobilization of the HSCs, which is crucial for peripheral blood stem cell transplantation (PBSCT). As anti-VLA-4 and anti-VCAM-1 monoclonal antibodies (MoAbs) are known to weaken the bonding between HSCs and stromal cells, the effect of anti-VLA-4 and anti-VCAM-1 MoAbs was evaluated on the mobilization of HSCs in mice. Materials and Methods. MoAbs to mouse VLA-4 and VCAM-1 and recombinant mouse G-CSF were used to improve mobilization. CD34 + cell counts, colony-forming units (CFU-C) in methylcellulose media culture and immunohistochemical staining for VCAM-1 were used to confirm mobilization in the PB and BM. To confirm the cleavage of bonds between the HSCs and the stromal cells, a long-term liquid culture with stromal cells was treated with these MoAbs. The cells were then photographed under a light microscope at various magnifications. Results. The percentage of CD34 + cells decreased in the BM and increased in the PB after anti VLA-4 and anti VCAM-1 treatment with doses of cytokines and MoAbs. CD34 + expression increased about 3-5 fold in each group compared to the untreated control group. In particular, there were large synergistic effects in the anti-VLA-4+G-CSF group. The CFU-Cs were visible in the PB mobilized with anti-VLA-4 and anti-VCAM-1. The CFU-Cs counts increased 7.73 fold in the G-CSF+ anti-VLA-4 combination group. A loss of adhesion was observed between clustered nests of hematopoietic and stromal cells after infusion of anti-VCAM-1 MoAb in the long term liquid culture media. We also found that positive immunohistochemical staining for VCAM-1 was abolished in mice with pre-treated anti-VCAM-1 MoAb. Conclusions. These results show that (1) Anti-VLA-4 and anti-VCAM-1 MoAbs may induce the mobilization of hematopoietic cells from the BM; (2) anti-VLA4 MoAb had a synergistic effect with G-CSF in terms of stem cell mobilization. Therefore, it may be possible to develop new methods for effective mobilization using these anti-VLA4 and anti-VCAM1 MoAbs for human peripheral stem cell transplantation.