It is known that between saliva and oral microorganisms there are close relations. To investigate the salivary factors that influence the metabolism of oral microorganisms, the production and degradation of indole were studied in several salivary supernatants or salivary ultrafiltrates. Washed salivary sediments were mixed with the supernatants or the ultrafiltrates containing tryptophan or indole. After incubation at 37 degrees C for 48 hours, the indole was extracted by petroleum ether and determined spectrophotometrically. The results obtained were as follows: 1) The amount of indole produced in the raw supernatants was generally greater than that produced in the heated supernatants. 2) The production and the degradation of indole in the ultrafiltrates varied according to the saliva samples. 3) Only a small amount of indole was produced when tryptophan was not added to the ultrafiltrates. 4) When the ultrafiltrate was preheated at 98 degrees C, the indole production slightly decreased for only ten minutes. 5) The optimum pH for indole production in the ultrafiltrate was within the range from 7.0 to 8.5 in the presence of 20mM of Good's buffer (MES, MOPS, TAPS). 6) The highest indole production was observed when MOPS (pH7.7) was added at the concentration of 20mM to the ultrafiltrate. 7) The amount of indole production was mainly influenced by salivary sediment, and it was also affected by the ultrafiltrate.