Considerable interest has been aroused concerning the effect of milk feeding on intestinal conditions, from the standpoint of change in intestinal flora and of clinical results. The observations of Metchinkoff and his contemporaries initiated this interest. Published reports of their experiments stimulated a number of investigators to further study of intestinal micro-organisms, particularly the group of gram-positive bacteria belonging to the genus concerned. These investigations show that persons living on a high milk diet often develop an intestinal flora which is composed mostly of grampositive rods of the B. acidophilus and B. bifidus types, more or less to the exclusion of the putrefactive and proteolytic types. Metchinkoff's observations were first made on the peasants of the Balkans. The main article of their diet is sour milk prepared by the fermentation of milk with a mixed culture of micro-organisms. As the predominating species in the sour milks studied by him and