The hemophilic bacteria of which Bacillus influenzæ serves as a type require for their growth two distinct and separable substances, both of which are present in blood and neither of which alone suffices. These substances are (a) a vitamine-like substance which can be extracted from red blood corpuscles, from yeast, and from vegetable cells, which is relatively heat-labile and absorbed from solution by certain agents; (b) a so called X substance which is present in red blood cells, is heat-stable, and acts in minute amounts.