Bovine dentin, bone, and tendon slices, and rat bone, readily mineralize to variable degrees after demineralization by (EDTA) at pH 7.4, but they fail to mineralize after demineralzation with acetic acid (HAc) at pH 3.0. The demineralized dentin, but neither bone nor tendon, contained organically bound phosphate. The EDTA-demineralized dentin contained less phosphate than HAc-demineralized dentin. HAc-demineralized rat dentin contained high levels of phosphate. Since the EDTA- and HAc-demineralized rat dentin contained widely different levels of phosphate, yet both mineralized, it was concluded that phosphoprotein had little effect on nucleation. The reason why HAc-demineralized tissues other than rat dentin failed to nucleate and mineralize was not clarified.