A method of improving the adhesion of an adhesive to the surface of hard tissue. Before the adhesive is applied, the hard tissue surface is primed with a polybiguanide or an addition salt thereof, or is pretreated with an aqueous solution of a magnesium salt and then primed with a primer selected from a long chain alkyl quaternary ammonium salt, a bisbiguanide, an acid addition salt of a bisbiguanide, a polybiguanide, and an acid addition salt of polybiguanide.