A study was conducted to evaluate the efficacy of the bonding agents supplied with three composite resins in improving the sealing ability of the materials to tooth structure and the bond of the resins to enamel. When the enamel was not etched prior to insertion of the resin, the use of the bonding agents did not improve marginal seal. Little or no marginal leakage was detected when the restorations were placed by means of the acid etch technique, either with or without use of the respective bonding agents. A slight improvement in the bond of the resins to unetched enamel was secured by use of the bonding agents; however, this advantage was lost when the specimens were thermal cycled. Acid etching of the enamel significantly improved the bond of the strength of all three resins, both when the specimens were thermal-cycled and when stored at a constant temperature. Only one of the commercial bond agents enhanced the bond of its respective composite resin to etched enamel subjected to cycling.