Freshly excised rat incisors were immediately cleaned and demineralized in 0.5 M ethylene diaminetetracetic acid at pH 7.5. The extracts were freed of calcium, diffusible phosphate and low molecular weight polypeptide components by dialysis in membranes with cut-off of 3500 molecular weight. The extract was resolved into at least 7 protein components by chromatography on DEAE-cellulose at pH 8.2. The composition of each protein component was determined. Two proteins, rich in serine, phosphorous and aspartic acid were unlike any proteins attributed to enamel, and hence were considered to be components of incisor dentin. These were the principal non-collagenous components of the teeth. Further purification was carried out under dissociative conditions on Sepharose CL-6B gel filtration columns in 3.0 M guanidine hydrochloride. The two phosphoproteins have mol wts, by this method, of 71,000 and 65,000, respectively, and differ in content of apolar amino acids, although both contain greater than 70 residue % of seryl (or phosphoseryl) and aspartyl residues. The name "phosphophoryns" is proposed to describe these dentinal proteins. The insoluble collagenous matrix remaining after the original demineralizing extraction was degraded with cyanogen bromide. Several non-collagenous protein components were released as well as the typical collagen derived peptides. Two collagen phosphoprotein complex peptides were also isolated, demonstrating as in bovine dentin, the probable direct covalent interaction of a dentin phosphoprotein with hte collagen of the mineralized matrix.