The ordinary method of making an emulsion of fæces in saline or broth and plating therefrom is of value in gross pathological infections of the bacillary and coccal variety. As a method of isolating the sum total of aerobic fæcal streptococci, it is, however, peculiarly incomplete. Anyone who cares to incubate a specimen of fæces in 1% neutral glucose broth and plate therefrom on successive days on to blood media will notice some very interesting facts. Whereas, in the majority of cases, coliform bacilli will grow therefrom in the first two or three daily platings, after a few days they cease to be recoverable and are replaced by streptococci of various types, sometimes later by staphylococci or moulds in pathological instances, and finally all that can be grown are Gram-positive bacilli or filaments. These bacilli are the lactic acid or acidophilus bacilli, and they have, by their production of acid and possibly by other lethal products, destroyed all aerobic bacilli except themselves. In time they, too, succumb, not apparently from hyperacidity, but from lack of pabulum. They can easily be kept alive by transplantation, to either human citrated blood-agar or glucose broth. Elsewhere∗ I am publishing a detailed account of this “acidophilus phenomenon,” but a consideration of the annexed Table I will make tolerably clear the mechanism of this curious process. It is, in the main, a question of acidity, inasmuch as any germ which produces a lower hydrogen-ion concentration (that is, a more acid medium) will inevitably destroy any accompanying germ which produces a less acid medium. For instance (see Table I), a mixture of B. coli communis and Staphylococcus aureus will eventually yield from glucose broth a pure culture of the latter organism, since B. coli produces a pH of not less than 4.0 in neutral glucose broth, whereas the staphylococcus can produce a pH of 3.8. Likewise a streptococcus producing a pH of 3.8 will oust a B. colt producing a pH of 4.2–4.0. We should thus expect an alkaline stool to yield a majority of coliform organisms, and a more acid stool to yield a majority of streptococci or staphylococci; this is precisely what happens.