A study of 58 strains of the lacto-bacillus group indicates that all of the cultures of L. acidophilus, all but 7 of L. casei, and single strains each of L. pentoaceticus and L. bifidus grew vigorously in media in which the surface tension had been depressed to 40.4 dynes. When the surface tension was reduced to 42.6 dynes, variable results were obtained with L. bulgaricus, but the other strains grew consistently. The author suggests the probable value of employing reduced surface tension, not only as a criterion of differentiation but as an index to certain metabolic processes of the cell which are not at present well understood.