Instillation of Bacillus Calmette Guerin (BCG) into the bladder is the standard treatment for superficial bladder cancer. It leads to a local inflammatory response due to the release of cytokines and influx of immune cells to the tumor site. Although the presence of an intact immune system is an essential criterion for successful therapy, attachment of the bacteria to the bladder urothelial is just as important. The purpose of our study is to determine the role of bacterial internalization by epithelial cells. Transfection of the alpha5 integrin gene into the BCG unresponsive bladder cancer cell line, RT4, caused an increase in bacterial uptake and also increased cell death. Treatment of cells with cycloheximide did not prevent bacterial internalization but blocked its cytotoxic effect suggesting that unlike cell death, the process of bacterial internalization does not require new protein synthesis. Our data also show that the bacteria secretory products can prevent its own internalization. The extract prepared from lyophilized BCG altered the phosphorylation status of the focal adhesion kinase which is responsible for cellular endocytosis. Therefore, bacterial phosphatases may be present in the bacterial extract. Their activity may inhibit BCG internalization. Thus washing the reconstituted bacteria to remove the enzymes before instillation into the bladder might improve the therapeutic outcome of intravesical BCG therapy.