The effects of addition of various copper salts to 2-hydroxyethyl methacrylate (HEMA) primer were studied with regard to tensile bond strength of methyl methacrylate-tributylborane (MMA-TBB) resin to dentin. Bovine dentin surfaces were treated with 10% phosphoric acid, then with a primer consisting of aqueous 35% HEMA and a copper salt, and finally bonded to acrylic rods with MMA-TBB resin. The types of copper salts and concentrations in the primers were varied. The optimal concentration of copper salts in the primers was 0.5-3 mumol/g. The primers containing copper salts of sulfate, methacrylate, methacryloyloxyethyl phthalate, or methacryloyloxyethyl succinate were especially effective among the eight copper salts studied in increasing bond strength, producing mean bond strengths of 14.2-16.1 MPa and mean minimum values of 9.9-11.7 MPa. These bond strengths were higher than those obtained with the commercial 4-META/MMA-TBB resin system. A model experimental study concerning the molecular weight of poly (MMA) formed in the presence of some copper salts suggested that the increase in bond strength was produced through an increase in molecular weight of MMA-TBB resin by the copper salts.