1. Bacteria may play but an insignificant part in the reduction of methylene blue in milk, though their de-oxygenating effect may be of influence in the commercial application of the test. 2. Milk as it exists in the udder, or milk drawn anaerobically, reducesmethylene blue almost instantaneously, whereas the same milk exposed to oxygen will usually take more than 10 hours to reduce. 3. The oxidation-reduction potential of anaerobically drawn milk is much lower than the same milk exposed to oxygen, and in accordance with its behaviour toward methylene blue. 4. Evidence is given for the presence of a redox system, present in low concentration, as responsible for the reduction of methylene blue. 5. Although the addition of small amounts of cysteine or glutathione to milk leads to the reduction of methylene blue, their absence from milk excludes them as possible factors in the normal reduction. 6. The possibility that lactoflavin may furnish the redox system is suggested. 7. The reduction of methylene blue in milk is catalysed by light in the visible spectrum.