1. Examine Endo’s medium or lactose-litmus-agar plates. If typical colonies have developed, select two and transfer each to a lactose broth fermentation tube and an agar slant, both of which are to be incubated at 37° C. 2. If no typical B. coli colonies are found, incubate the plates another 24 hours. _Fifth Day_: 1. Select at least two colonies, whether typical or not, from the Endo’s medium or lactose-litmus-agar plates which have been incubated an additional 24 hours; transfer each to a lactose broth fermentation tube and an agar slant, and complete the test as for typical colonies. 2. Examine lactose broth fermentation tubes inoculated from plates on the previous day. Tubes in which gas has been formed may be discarded after the result has been recorded. Those in which no gas has formed should be incubated an additional 24 hours. _Sixth Day_: 1. Examine lactose broth fermentation tubes reincubated the previous day. 2. Examine microscopically agar slants corresponding to lactose fermentation tubes inoculated from plate colonies and showing gas-formation. BACTERIOLOGICAL BIBLIOGRAPHY. Bibliography 201: BOVIE, W. T. A Direct Reading Potentiometer for Measuring and Recording both the Actual and the Total Reaction of Solutions. _Jour. Med. Research_, 33, 1915–16, 295. Bibliography 202: BREED, R. S. and DOTTERRER, W. D. The Number of Colonies Allowable on Satisfactory Agar Plates. _Jour. of Bact._, 1, 1916, 321. Bibliography 203: BROWNE, W. W. A Comparative Study of the Smith Fermentation Tube and the Inverted Vial in the Determination of Sugar Fermentation. _Amer. Jour. of Public Health_, 3, 1913, 701. Bibliography 204: CLARK, W. M. An Hydrogen Electrode Vessel. _Jour. Biol. Chem._, 23, 1915, 475. Bibliography 205: CLARK, W. M. The “Reaction” of Bacteriological Culture Media. _Jour. of Inf. Diseases_, 17, 1915, 109. Bibliography 206: CLARK, W. M. The Final Hydrogen Ion Concentrations of Cultures of Bacillus Coli. _Science_, n. s. 42, 1915, 71. Bibliography 207: CLARK, W. M. and LUBS, H. A. Hydrogen Electrode Potentials of Phthalate, Phosphate, and Borate Buffer Mixtures. _Jour. Biol. Chem._, 25, 1916, 479. Bibliography 208: CLARK, W. M. and LUBS, H. A. The Differentiation of Bacteria of the Colon-Aërogenes Family by the Use of Indicators. _Jour. of Inf. Diseases_, 17, 1915, 160. Bibliography 209: ENDO, S. Ueber ein Verfahren zum Nachweis der Typhusbacillen Centbl. f. Bakt. _Erste Abt._, 35, 1903–4, 109. Bibliography 210: GILLESPIE, L. J. The Reaction of Soil and Measurements of Hydrogen Ion Concentration. _Jour. Wash. Acad. of Sciences_, 6, 1916, 7. Bibliography 211: HILL, H. W. Porous Tops for Petri Dishes. _Jour. Med. Research_, 13, 1904, 93. Bibliography 212: