OBJECTIVE The effects of time, surface preparation, and use of a bonding agent on the ability to bond amalgam or resin composite to amalgam were studied. METHODS Cylindrical amalgam specimens were prepared in a split die (32 groups of 10 each). The 32 groups were divided into two groups of 16 each: amalgam bonded to amalgam and resin composite bonded to amalgam. One half of each group of 16 was bonded within 1 hour after the initial condensation (fresh) and the other half was bonded after 21 days' storage in physiologic saline (old). Four bonding methods (control [no bonding agent], Advance, All-Bond 2, or Amalgambond Plus) were each utilized on one abraded group (air abrasion with 50-microns aluminum oxide) and one unabraded group. After an additional 21 days of storage, bond strengths were measured with a three-point flexure test. Solid amalgam cylinders also tested. RESULTS Solid amalgam control cylinders exhibited mean bond strengths 10,000 psi greater than any of the bonded specimens. Among specimens of amalgam bonded to amalgam, those that were abraded after 21 days and used no bonding agent showed a significantly greater bond strength than did specimens utilizing any of the other combinations of the selected independent variables. Among specimens of resin composite bonded to amalgam, those bonded after 21 days and utilizing a bonding agent exhibited significantly greater bond strengths than did specimens utilizing any of the other combinations of the selected independent variables. CONCLUSIONS The results of this in vitro study suggest that the strength of a repaired amalgam is only about 40% of that of an unrepaired amalgam. They also indicate that the strongest repair of amalgam using additional amalgam material may be accomplished without the use of any bonding agents utilized in this study. When adding resin composite material to amalgam, the resin composite should be added after the amalgam has had adequate time to set, and the use of a bonding agent increases the strength of the repair.