1. In good quality raw milk reducing methylene blue at 37–38° C. members of the coliform group andStaph. aureuswere among the commonest organisms present at the time of reduction. Other types of staphylococci, micrococci, and certain streptococci were not infrequently present in smaller numbers. In any given milk one type of organism tended to assume predominance.2. Using the modified test described by Wilsonet al.(1), various organisms isolated from milk at the time of reduction were examined for their rate of decolorization of methylene blue when growing in pure culture in raw “sterile” milk (milk freshly drawn from the healthy udder under aseptic conditions and having a plate count on milk agar at 37° C. of 200–300 per ml.). The most rapidly reducing organisms belonged to the coliform group. Following them, in decreasing order, cameStr. lactisand some of the faecal streptococci,Staph. aureus, Staph. albus, Staph. citreus, some micrococci, group C haemolytic streptococci and some strains ofStr. agalactiae, and aerobic spore-bearers