Litmus milk is probably the most useful medium to the dairy bacteriologist, since it indicates fairly accurately what will be the action upon milk of the micro-organism under investigation. In spite of the advantages of synthetic indicators, such as brom-cresjol-purple, litmus is still used because it gives a greater variety of reactions than a synthetic indicator. Thus the reducing power of a culture is adequately indicated by litmus, the normal E being about –0·09 volt at pH 5(1). Some lactic acid bacteria, e.g. Str. lactis, readily reduce litmus, while others do not or only very slightly, e.g. Sbm. plantarum. Plain litmus milk is, however, an inadequate medium for some bacteria, which either clot the medium only after several weeks growth, or never produce sufficient acid to bring about this change (2).