Semisynthetic medium, containing vitamin-free casein acid hydolysate as nitrogen source, is generally superior, in growth-promoting ability, to pure synthetic medium, containing amino acids and inorganic salts approximately in the same composition as the former. According to the reports of Demain, it was assumed that a minute amount of fatty acid, which is present in hydrolysate, would exert growth-promoting action. In order to ascertain this assumption, casein acid hydrolysate was treated with ether, and the ether-soluble portion was examined for growth-promoting action. As the result, in ether-soluble portion of vitamin-free Casamino acids (Difco), it was found a factor which exerted accelerative effect on the growth of Streptococcus hemolyticus S8, though not Escherichia coli, Salmonella pullorum, Vibrio El Tor, Brucella melitensis, Staphylococcus aureus and Diplococcus pneumoniae. Paper chromatography demonstrated that about 0.01-0.001% stearic acid was contained in Casamino acids. In view of this, pure fatty acids were investigated for growthpromoting effect on Streptococcus hemolyticus S8, and palmitic, stearic and arachidic acid were found effective. Especially, stearic acid was most effective, promoting the bacterial growth even in concentrations below 1.0γ/ml. Oleic, linoleic and 1 inolenic acid did not show any accelerative effect, and in higher concentrations, all of them were conversely inhibitory. It was confirmed inthis way that vitaminfree Casamino acids contained a minute amount of saturated fatty acid which promoted the growth of Streptococcus hemolyticus S8.