Thirty-six extracted, noncarious, nonfractured human incisors were divided into four groups of nine teeth. Endodontic access cavities were prepared, the pulp chamber was debrided, the root canals were cleansed, and root canal treatment was completed. Pulp cavities of teeth in group 1 received a cotton pellet and were sealed with Cavit. Groups 2, 3, and 4 received a mixture of 30% hydrogen peroxide and sodium perborate for 3, 4, and 7 days, respectively, were sealed with Cavit, and were stored in a humidor until used. Cavit and the other materials were removed, and the cavities were rinsed and restored with Scotchbond Multipurpose and Silux. The teeth were thermocycled, stained with 50% silver nitrate, and sectioned longitudinally. Dye penetration was measured. Results indicated that bleaching adversely affected the marginal seal at the tooth-restoration interface, as evidenced by increased microleakage; the highest rate of microleakage was found after the 7-day application of bleaching materials.