Root conditioning/biomodification in periodontal therapy has been shown to cause dentin root resorption. Dentin resorption is thought to be induced by adhesion of osteoclasts onto the root surface, demineralization and subsequent degradation of the organic matrix, which, is predominantly composed of type I collagen. Objectives: The purpose of this study was to evaluate the effects of collagen cross-linking induced by glutaraldehyde (GA) and two naturally occurring cross-linkers: proanthocyanidin (PA) and genipin (GE) on its enzymatic digestibility. Methods: The upper third of the root dentin prepared from three human molars was pulverized and demineralized with EDTA. Aliquots (~2mg) were treated with the following cross-linkers: 5% GA, 0.5% PA or 0.625% GE for 72 hours at room temperature. As controls, aliquots were incubated in PBS or left untreated for 72 hours at room temperature. After the treatments, the samples were extensively washed with distilled water and digested with bacterial collagenase for 24 hours. After the digestion, the samples were centrifuged and the supernatants were lyophilized, hydrolyzed with 6N HCl and subjected to amino acid analyses to determine hydroxyproline, and the residues to 6% SDS-PAGE. Results: The amino acid analyses revealed that while almost all collagen was dissolved by collagenase digestion in the controls, only ~10%, 20% and 2% of collagen were solubilized in the GA-, GE- and PA-treated groups, respectively. There were no discernible protein/peptide bands on the gel in the controls but the treated groups showed smear bands some of which may correspond to a1 and a2 chains of type I collagen. Conclusions: This preliminary study indicates that the two naturally occurring cross-linkers tested, i.e. proanthocyanidin and genipin, could be used to modify root dentin collagen to efficiently increase its resistance against enzymatic digestion, thus, preventing its degradation during root resorption. Supported by NIH-DE10489 and DE16116-01.