SUMMARY: The effects of shaking milk samples before the addition of methylene blue, delaying the time of addition of the dye, adding a second quantity of dye with a hypodermic syringe after the initial charge had been decolourized and of increasing the storage time or varying the testing time have been studied in five separate experiments. Vigorous shaking at the time of dye addition lengthened the reduction time. This effect was more pronounced when the milk was stored in a tube than in a bottle. Delayed addition of the dye resulted in a shorter reduction time than expected for inverted samples and longer than expected for samples which were not inverted between the time they were put into the 37° bath and the addition of the dye. Second additons of dye did not usually have reduction times exceeding 1 hr. Methylene blue reduction times did not necessarily decrease with successive tests carried out over a period of time on one sample: a time of 30 min. might persist over a series of tests for 4 hr. or more and small increase and decrease in reduction time with successive tests were also noted.