Objective: There are concerns about the possible pathologic effects of relatively long-term exposure of oral tissues to bleaching agents. The purpose of the present study was to investigate the cytotoxic effects of various bleaching agents on mouse mammary carcinoma FM3A cell line. Method and materials: FM3A cell line obtained from European Collection of Animal Cell Cultures was used in cell culture assay. Exponentially growing cells were seeded in 1 x 10⁵ cells/mL in 5 mL of RPMI 1640 medium supplemented with 10% fetal calf serum and antibiotics in each well of a six-well plate. PowerGel was applied onto the cell culture medium and incubated for 24 hours at 37°C in a 5% carbon dioxide atmosphere after light curing. Opalescence PF gel was left on cell culture medium for 24 hours at the same conditions. After 24-hour incubation, the cells were collected by trypsinization and counted with a hemocytometer. For cell viability, trypane blue exclusion assay was used. The cytotoxicity of PowerGel and Opalescence PF were determined by evaluation of cell growth and viability in comparison to untreated controls (cell growth = 100%). The data were submitted to statistical analysis (One-way analysis of variance, Dunnett's t test). Results: Cell growth after 24 hours was significantly reduced (P = .000) in comparison to the control group. The cell growth in the Opalescence PF gel group also was reduced (P .05). It was found that both gels affected cell viability. Conclusion: PowerGel and Opalescence PF showed a cytotoxic effect on cell growth in FM3A cell line.