1. The mixed evening and morning milk from 360 cows, all sampled six times during their lactation, was analysed chemically and for its cell content. A total and a differential cell count were made. Similar analyses were made on 823 samples of farm milk taken at a receiving creamery.

2. The total cell counts varied from less than 10,000 to 5,000,000/ml. and there was considerable variation in the chemical composition of samples containing similar numbers and types of cells.

3. The proportion of polymorph cells in the total count increased rapidly with increasing total count.

4. Stage of lactation had little effect on the number of cells present in the milk, but samples from cows in very early and very late lactation had been avoided. The total cell count was lowest from the 70th to 130th day of lactation with a tendency to rise after the 130th day.

5. Both the total and the polymorph cell counts of the milk increased sharply with the lactation number of the cow until the seventh lactation and then remained at a fairly constant level.

6. As the total cell count increased to an average value of 500,000/ml. there was a fall in the daily yield of milk per cow and a marked fall (0·25%) in solids-not-fat content. The latter was the result of a steady drop in the lactose content, the casein content being affected only by counts approaching 1,000,000/ml. An increasing cell count was without effect on the fat content. Possible reasons for the relatively small effect on chemical composition of high cell counts in some milks is discussed.

7. It is concluded that the presence of subclinical mastitis, as evidenced by the cell count of the milk, leads to a lowering of the solids-not-fat content.

The authors wish to thank their assistants in the Technical Chemistry and Veterinary Pathology Departments, who did most of the practical work.