Bacterial contamination of dentin may be a contributing factor in the development of bleaching-associated root resorption. In this study, the effect of commonly used bleaching agents on the permeability of dentin tubules to Streptococcus faecalis was evaluated. Sixty extracted bovine incisors were horizontally sectioned apically to the cementoenamel junction. In each tooth, a standard cavity was prepared, the pulp tissue extirpated, and remnants of soft tissue and smear layer were removed. Following rinsing and repeated autoclave sterilization, the teeth were divided into four groups, each treated with one of the following materials: 30% hydrogen peroxide, sodium perborate mixed with 30% hydrogen peroxide, sodium perborate mixed with distilled water, and distilled water alone that served as control. The bleaching agents were sealed in the teeth and incubated at 37 degrees C for 7, 14, and 21 days. At each time interval the bleaching agents were removed and the teeth incubated at 37 degrees C in brain heart infusion infected with S. faecalis. Histological sections were prepared, and the maximal bacterial penetration for each group was measured using a computerized morphometric system. Statistical analysis of the results revealed that teeth treated with either 30% hydrogen peroxide alone or in combination with sodium perborate were significantly more permeable to S. faecalis than those treated with sodium perborate mixed with water (p < 0.0001). Sodium perborate mixed with water did not cause an increase in dentin permeability to S. faecalis and was similar to the water control. In conclusion, it seems that bleaching agents containing hydrogen peroxide in high concentrations may increase bacterial penetration through dentinal tubules.