1. Bull calves from tuberculin-tested Ayrshire herds were fed on raw or commercially pasteurized milk up to 12 weeks of age, in amounts strictly in relation to their body weight. This diet was supplemented from the eighth week by hay at the rate of 2/3 lb. per head per day.

The milk used was mixed milk from untested herds, the raw and pasteurized milk being derived from the same bulk sample. The quality of the milk was controlled by daily butterfat and phosphatase tests and by inoculation of grouped daily aliquots twice weekly into duplicate guinea-pigs.

2. The total number of calves used in the experiment was ninety-two. Of these, forty-two received pasteurized milk and fifty raw milk. Nineteen calves died before completion of the experiment, five in the pasteurized milk-fed group and fourteen in the raw milk-fed group. The mean ages of these calves at death were 44 and 27 days respectively.

Seventy-three calves completed the full 12 weeks of the experimental feeding. Of these, thirty-seven received pasteurized milk and thirty-six raw milk. The former group showed a mean percentage gain in live weight of 80·2 ± 3·1, the corresponding figure for the latter group being 74·5 ± 2·7. The difference, i.e. 5·7 + 4·1, was not significant. Reasons are given for the relatively low growth rate observed.

3. No appreciable differences were noted in the skeletal growth of the two groups. Marks awarded by experienced stock judges showed consistent differences in favour of the pasteurized milk-fed group, although the significance of such differences cannot be assessed.

4. Inoculations of grouped daily aliquots of raw milk twice weekly into duplicate guinea-pigs resulted in finding viable tubercle bacilli in 70% of the samples and Br. abortus in 38% of them. The pasteurized milk samples were uniformly negative to both tests.

The differences in tuberculous infection of the two types of milk were reflected in the results of tuberculin tests and post-mortem examinations on the calves at the conclusion of the experiment. Twenty-four out of thirty-six calves fed on raw milk reacted to the test, and the presence of tuberculous lesions was confirmed in twenty-three by post-mortem examination. One calf in the pasteurized milk-fed group reacted to the test, but exhaustive post-mortem examination and inoculation of glandular material into guineapigs failed to confirm the presence of any tuberculosis.

5. This work has failed to show any significant differences in the nutritive value of raw and of pasteurized milk for the rearing of young calves. The use of pasteurized milk, however, had a clear advantage in that it preserved the animals from infection through drinking milk containing living tubercle bacillli.