The yeast growth stimulant produced during the growth of Aspergillus niger on sucrose media was found to be similar to that obtained from the extract of malt sprouts. Bios II prepared from the mold-produced stimulant by means of alcoholic barium hydroxide and Bios II from malt sprouts had similar effects upon the growth of yeast. The results obtained by Schopmeyer (1931), to the contrary, may be explained on the difference in the media and the yeast employed;The importance of magnesium sulfate in media used in bios studies was emphasized. The effect of the concentration of magnesium sulfate upon the growth of various strains of Saccharomyces cerevisiae in the presence of Bios II and inositol was determined. With all the strains studied, the addition of magnesium sulfate to Medium C increased the stimulative effect of Bios II and inositol;The effect of inositol upon the stimulation of yeast growth by Bios II was studied. The complementary effect of inositol is dependent upon the strain of yeast employed, and upon the concentration of magnesium sulfate. In the absence of inositol the higher concentrations of magnesium sulfate are toxic toward certain strains of yeast. In such cases the addition of inositol gave large increase in growth;Twenty strains of Saccharomyces cerevisiae were studied in regard to growth in the presence of inositol, Bios II, and magnesium sulfate. The strains were divided into four groups according to their reactions to various combinations of the three substances;Photomicrographs were made of four different strains to show the ability of inositol to reduce the clumping of cells;Comparison was made of the growth of various strains of yeast in the media of Clark, Williams, and Fulmer in order to determine the suitability of each for use in bios studies.