Oxidative stress is a consequence of an imbalance between reactive oxygen species production and antioxidant defence mechanisms. Although the cell, exerts different antioxidant defence mechanisms, oxidative damage in cellular structures at high concentrations of ROS is still present, and it plays an important role in the development of many diseases. Besides its basic role in inhibition of cathepsins, stefin B has an important role in reducing oxidative stress in cells. In our work we tested whether the silencing of stefin B gene would increase sensitivity to oxidative stress in human cancer lines. We prepared three different short hairpin RNA sequences for silencing stefin B gene, and one control short hairpin RNA sequence. We did not get suitable results using a stable transfection. With a transient transfection, we managed to partly silence stefin B gene in both human cancer lines and checked cell death of transfected cells due to the H2O2 - induced oxidative stress. Results showed that partial silencing of stefin B gene is not enough to increase the sensibility of cells to the H2O2 - induced oxidative stress.