['To determine whether Bacillus Calmette-Guérin (BCG) and/or BCG-soluble factors could modulate cellular reactive oxygen species (ROS) in human bladder cancer cells and the impact this could have on response to therapy.', 'The expression of α5β1 integrins on human bladder cancer cell lines and their ability to internalize BCG were determined. The effect of live and lyophilized BCG on cellular ROS, lipid peroxidation, and DNA damage was determined using H(2)DCF-DA, TBARS, and comet assays. The cytotoxic effects of live and lyophilized BCG on cancer cells were determined after 24 hours. ROS modulation by Antigen 85B and mycobacterial protein tyrosine phosphatases was monitored.', 'Live and lyophilized BCG were internalized to a similar extent, but live BCG increased cellular ROS, whereas lyophilized BCG reduced ROS. High ROS levels correlated with increased lipid peroxidation. The cytotoxic effect of BCG was independent of cellular ROS but dependent on internalization. Lyophilized BCG was more cytotoxic to bladder cancer cells than live BCG. BCG soluble factors such as Antigen85B could increase cellular ROS. Internalization of lyophilized BCG abrogated the ROS, and lipid peroxidation increase induced by BCG soluble factors. Both live and lyophilized BCG induced DNA damage but to different extents.'] Does bacillus Calmette-Guérin induce cellular reactive oxygen species and lipid peroxidation in cancer cells?