Summary 1. Five healthy, newborn male infantswere fed diets consisting of “humanized” cow's milk alternated with pooled human breast milk for total periods of nine to twenty-seven days, with an average duration of about fifteen days on each type of feeding. 2. Determinations of fat balance,stool pH, and bacterial flora, including L. bifidus var. pennsylvanicus , are reported. 3. Contrary to the results of previously reported studies, (a) there was no significant difference in percentage of fat retention on the two types of feedings; (b) the fecal pH and relative numbers of gram-positive rods in the feces were similar for both groups; (c) L. bifidus var. pennsylvanicus was present in the stools during the periods in which the infants received breast milk and in some periods on “humanized” cow's milk immediately following breast milk feeding. This organism was diminished in periods in which feeding of the modified cow's milk formula preceded breast milk feeding.