Publisher Summary The growth of C. diphtheria occurs on the ordinary media, but it is more profuse on enriched media. One of the most satisfactory media in use is Loeffler's serum, which is prepared by adding 1 part of 1% glucose broth to 3 parts of sterile ox-serum and inspissating. The optimum temperature is 37° C., range 14°–40° C, and the optimum reaction is pH 7.2. The diphtheria bacillus is a facultative anaerobe, but only scanty growth is obtained in the absence of free oxygen. C. diphtheria exhibits little proteolytic activity. Growth takes place on gelatin without liquefaction of the rnedium; indole is not produced. Several carbohydrates are fermented with the production of acid only. In these tests, Hiss's serum—water medium is employed and important sugars are glucose, maltose and saccharose; the two former are fermented, but there is never any fermentation of saccharose. This is a point of differential importance, as many, but not all, diphtheroids ferment saccharose.