1. A simple method of determining the acid degree of cheese fat has been described.

2. An average value of fat acidity on a series of thirty-eight samples of experimental Canadian first-grade Cheddar cheese, covering a period of about a year, has been eatablished at 1·0 ml. N/10 NaOH per 10 g. for cheese 2–4 weeks old.

3. An average acid degree for four experimental cheese from pasteurized milk was found to be 0·6.

4. A high acid degree was obtained on cheese fat made from milk in which lipase action was induced by vigorous agitation; the longer the period of agitation the higher was the acid degree. Extensive lipolysis took place at some stage before the cheese was removed from the press. Subsequent increase in acid degree was slight.

5. Fat acidities on a series of commercial Cheddar cheese, selected so as to include a high proportion of cheese degraded for unclean and rancid flavour, ranged from 1·2 to 4·6 with two exceptionally high values of 24·8 and 26·5. The incidence of undergrade cheese increased as the acid degree increased. All cheese with an acid degree of 2·8 or higher were below first grade on flavour.

6. Cheese made from milk, in which acid was developed by the addition of a small amount of starter the night before and which was partly homogenized, showed a high acid degree although the flavour was quite good—often first grade. Similar cheese, but with 3% starter added in the morning, showed only slightly higher fat acidity and scored lower for flavour. These findings cannot be explained as a simple inhibition of the action of milk lipase by acid development.