The causes of acid liquefaction in a high grade pasteurized milk were investigated.

1. Liquefying milk produced much less acid and had a much lower plate count than normal milk.

2. Bacillus albolactis was found regularly in the milk. It made up a much larger percentage of the total flora in liquefying than in normal milk.

3. Associated growth experiments showed that Strep. lactis could control the liquefying activities of Bacillus albolactis.

4. Pasteurization experiments showed that milk heated for 20 minutes at temperatures higher than 61°C. would undergo acid liquefaction. This appeared to be due to the increase in the percentage of Bacillus albolactis.

5. Inoculation of pasteurized samples with Strep. lactis prevented acid liquefaction.

6. Comparison with a type species showed that the strains of Strep. lactis repeatedly isolated from the milk studied were of the tardus variety.

7. The liquefaction was probably primarily due to an increased percentage of Bacillus albolactis brought about by pasteurization; the lack of the type of species of Strep. lactis and the absence of lactobacilli might also have been contributing factors.