Mild oxygenating agents generatinglow concentrations of hydrogen peroxide (H2O2) are effective alternatives to heat‐activated 30% H2O2in bleaching discolored, vital teeth. There are concerns about possible pathological effects of long‐term exposure to bleaching agents, and irritation and ulceration of the gingiva and other oral soft tissues can occur. The objective of this study was to determine the effect of one of these agents on gingival fibroblasts in vitro. Microscopic examination revealed that concentrations of 0.05% to 0.025% of the agent appeared to kill most of the cells. At concentrations of 0.025% to 0.017% some morphological changes were noted; the cells appeared normal at concentrations of ≤0.0125%. The agent significantly (P≤0.002) decreased proliferation (measured by incorporation of [3H]‐thymidine into cellular DNA) at concentrations as low as 0.006%. The agent also had a dose‐dependent effect on fibronectin production, measured by ELISA, causing significant (P≤0.03) decreases at concentrations as low as 0.017%. The agent significantly decreased the production of types I (P≤0.01) and III (P≤0.04) collagens (measured by ELISA) at concentrations as low as 0.0125%. Type V collagen was not detected under any conditions. Catalase, which catalizes the breakdown of H2O2, abolished toxic effects of a 0.05% solution. The results show that in vitro, the agent is toxic to human gingival fibroblasts, inhibiting several cellular functions. Taken together, the findings of the study suggest that while the agent is toxic to gingival fibroblasts in vitro, enzymes in the oral environment which destroy H2O2may protect oral tissues and their component cells in vivo from the potential toxic effects of the agent.J Periodontol 1995; 66:7–13.