Standard sized cylinders of a small particle light-cured resin were bonded to the flattened labial surface of young bovine incisor teeth which had been previously subjected to four different treatments: (a) immersion in 35% hydrogen peroxide and etched with 37% phosphoric acid gel for 60 s, (b) immersion in saline and etched for 60 s, (c) etched for 60 s and immersion in hydrogen peroxide, and (d) etched for 60 s and immersion in saline. Two hydrogen peroxide and saline immersion periods were used, 5 and 30 min. Specimens were stored in water at 37 degrees C for 1 and 7 days before tensile and shear testing. A total of 256 teeth were used, 8 for each treatment group, each immersion period, and each water storage period for each test. Statistical analysis of the results indicated that there was a highly significant reduction in the adhesive bond strength of the resin when the enamel was exposed to hydrogen peroxide and that the reduction was, within the limits of this study, time dependent. The bond strength was unaffected by the etching order and the period of water storage. Scanning electron microscopic examination of randomly selected fractured peroxide-treated specimens indicated that the failure occurred primarily at the bonding resin-enamel interface and that it was associated with areas of resin nonattachment and an alteration in resin quality. It is suspected that these changes were caused by the presence of residual peroxide or peroxide-related substances at or near the enamel surface.