Free neutral D-amino acids have previously been detected in human plasma, usually accounting for less than 2% of the total free amino acid concentration (D-amino acid ratio) [Nagata, Y., Masui, R., Akino, T., 1992a. The presence of free D-serine, D-alanine and D-proline in human plasma. Experientia 48, 986-988. Nagata, Y., Yamamoto, K., Shimojo, T., 1992b. Determination of D- and L-amino acids in mouse kidney by high-performance liquid chromatography. Journal of Chromatography 575, 147-152. Nagata, Y., Yamamoto, K., Shimojo, T., Konno, R., Yasumura, Y., Akino, T., 1992c. The presence of free D-alanine, D-proline and D-serine in mice. Biochimca et Biiophysica Acta 1115, 208-211]. In the present study to search for the source of free D-amino acids, D- and L-enantiomers of the major non-essential amino acids, i.e., the free form of serine, alanine, proline, aspartate and glutamate were analyzed by HPLC in human saliva, submandibular glands and oral epithelial cells. The D-enantiomer ratios to total of free alanine or proline were 35% and 20%, respectively, in saliva. The ratios of the other D-amino acids were less than 11%. The effect of ingested food and oral bacteria on the saliva amino acid levels was suggested to be insignificant. D-Alanine and d-aspartate were also detected in the submandibular gland in ratios up to 5%, and D-alanine and d-proline were found in oral epithelial cells in ratios of 18% and 5%, respectively. The submandibular gland and oral epithelial cells are suggested to be possible sources of the saliva D-alanine and D-aspartate.