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.