The importance of Lactobacillus acidophilus in infancy and its therapeutic use in later life are well established.1 Since intestinal flora of healthy weaned infant is predominantly L. acidophilus, question may be raised as to what becomes of these indigenous organisms. Surprisingly enough there is very little in literature on which to base any notion as to incidence of L. acidophilus in adult intestinal tract. One of us (Kopeloff) and Cohen2 reported L. acidophilus present in 27 per cent of 45 healthy young medical students. Kulp 3 found L. acidophilus in 2 of 8 cases, or 25 per cent. Goldman4 observed presence of gram-positive nonspore-bearing and spore-bearing aciduric bacilli in 1 of 4 normal persons (25 per cent) and in 49 per cent of hospital patients. Vassiliadis 5 gives incidence of L. acidophilus in man as 5 per cent. More recently as a result of a study of 26 patients in a psychopathic hospital, Sanborn 6 concluded that the majority of persons on an ordinary diet have L. acidophilus present in their feces to an appreciable extent, but that at two extremes are a group in which L. acidophilus is practically absent and a group in which it is present, at least in some specimens, to