Velcade (also known as PS-341 or Bortezomib) is a highly selective and reversible inhibitor of the 26S proteasome and is approved for the treatment of patients with advanced multiple myeloma. Here we investigated the anti-proliferative effect of Velcade on 4T1 breast cancer and B16F10 melanoma cells and evaluated the mechanism of action. It was found that two cell lines are differentially sensitive to proteasome inhibitor Velcade. The IC50 concentrations for B16F10 and 4T1 were 2.5 nM and 71 nM, respectively. indicating that B16F10 cells are more sensitive to proteasomal inhibition. Velcade was equally potent in inhibiting the chymotrypsin-like activity of the proteasome in both cell lines. It was determined that B16F10 cells proliferate more rapidly than 4T1 cells; doubling time (T-d) = 14.2 h versus T-d = 22.9 h, suggesting that a rapid proliferation rate may be an important factor in cellular resistance towards proteasomal inhibition. We observed for the first time that p53 and p21 proteins were increased in B16F10 cells but not in 4T1 following Velcade-treatment, demonstrating that p53 and p21 may enhance Velcade sensitivity. Furthermore, it was observed that caspase-3 proenzyme was reduced by similar to 20% in B16F10 melanoma cells, but not in 4T1 cells in response to 26S proteasomal inhibition by Velcade. Altogether, we concluded that p53 protein plays a central role in higher sensitivity of B16F10 cells to Velcade by inducing the accumulation of p21, a cell cycle inhibitor, as well as by stimulating the mitochondrial pathway of apoptosis through caspase-3 activation.