During the years 1903 and 1904 while engaged upon an investigation of the micro-organisms present in saliva, I had frequent occasion to make a quantitative bacteriological examination of this material. The object then in view was to define the bacteria that are most numerous in the mouth, so as to see if they could be applied to detect particulate pollution of air by material derived from the upper respiratory passages in a way similar to that in which B. coli is used to detect and measure excremental pollution of water. While certain streptocoeci were found to provide the index in question, particularly S. salivarius (which is constantly present to the extent of 10 to 100 millions per cubic centimetre of normal saliva) it was observed that among other bacteria present in the cultures were certain gram-negative cocci which frequently exceeded 100,000 per c.c. of the saliva of normal individuals.