Cylinders of a visible light-cured microfil resin were formed on and bonded to the flattened enamel surface of 15 human hemisected premolar teeth which had previously been subjected to three different treatments: (a) immersion in 35% hydrogen peroxide (HP) for 60 min, (b) immersion in 35% HP for 60 min followed by storage in distilled water for 1 day prior to resin application, and (c) immersion in saline (S) for 60 min. Specimens were stored in distilled water at 37 degrees C for 7 days prior to shear bond strength testing. A total of 30 specimens were tested. Statistical analysis of the data indicated that there was a highly significant reduction in shear bond strength between HP- and saline-treated specimens. Water storage of HP-treated specimens for 1 day prior to resin application appeared to restore the adhesiveness but not to a point that was statistically significant. Scanning electron microscopic examination of randomly selected, fractured test specimens indicated that the reduction in bond strength may be related to alterations in the ability of the resin to attach itself to the HP-treated surface and to possible effects of the HP on the resin itself.