In order that results as reported may be checked and carefully valuated, it is necessary that the report should show not only the average number of B. coli per cc., but also the number of samples examined; and, for each dilution, the total number of tests made, and the number (or per cent.) positive. 10. INTERPRETATION OF RESULTS. While it is not within the province of this report to suggest the proper interpretation of results obtained by the use of the methods herein specified as standard, the committee feels that a word of caution should be given regarding the significance of the presence in a water of members of the B. coli group as defined in this report. Recent work seems to indicate that the B. coli group as herein defined consists of organisms of both fecal and non-fecal origin. Therefore care must be exercised in judging the sanitary quality of a water solely from the determination of the presence of members of the group. 11. DIFFERENTIATION OF FECAL FROM NON-FECAL MEMBERS OF THE B. COLI GROUP. (1) At least 10 cultures should be used. If possible these should be subcultured from plates made direct from the water since all of the cultures obtained by plating from fermentation tubes may be descendants of a single cell in the water. If cultures from water plates are not available those obtained from plates made as prescribed under B (p. 101) may be used. (2) Inoculate each culture into dextrose potassium phosphate broth,[H] adonite broth, and gelatin. For additional confirmatory evidence inoculation may be made into tryptophane broth,[I] and saccharose broth. The dextrose broth must be incubated at 30°. Other sugar broths may be incubated at 30° or 37° as convenient. Gelatin should be incubated at 20°. Footnote H: (a) _Peptone Medium for the Methyl Red Test. To Make One Liter._ 1. To 800 cc. of distilled water add 5 grams of Proteose-Peptone, Difco., or Witte’s Peptone (other peptones should not be substituted), 5 grams c. p. dextrose, and 5 grams dipotassium hydrogen phosphate (K_{2}HPO_{4}). A dilute solution of the K_{2}HPO_{4} should give a distinct pink with phenolphthalein. 2. Heat with occasional stirring over steam for twenty minutes. 3. Filter through folded filter paper, cool to 20° C. and dilute to 1,000 cc. with distilled water. 4. Distribute 10 cc. portions in sterilized test-tubes. 5. Sterilize by the intermittent method for 20 minutes on three successive days. Footnote I: _Tryptophane Broth for Indol Test._