Background: Eosinophilic round bodies (ERBs) are composed of fragments of amino acid sequences of varying lengths, which represent a portion of exon 5 of amelogenin gene. ERBs have previously been shown to accumulate in the backs of rats subcutaneously injected with the product Emdogain^®. In the present study, an oligopeptide with an amino acid sequence similar to this portion was artificially synthesized, and its effects were examined. Methods: The peptide solution was applied to periodontal defects created on the maxillae of rats. Sections from these tissues were histopathologically and immunohistochemically examined at 7 and 14 days after the start of the experiment. Results: In the experimental group, hard tissue had formed on the surface of exposed dentin, and both osteopontin and collagen type I (COLI) were localized around the hard tissue. The hard tissue was stained with periodic acid-Schiff (PAS) more deeply than was the dentin, and the fibers that were positive for COLI were vertically inserted in the rim of the newly-formed hard tissue. No hard tissue formation was found in the control group. Conclusion: These results suggest that synthetic oligopeptide administration can induc