1. In surveys carried out in the summer of 1951, 1·4% of churn milk contained penicillin in one group of tests and 2·8% in another. Bulk milk was free from this antibiotic.

2. In a further survey carried out in the winter of 1953, 3·2% of composite churn milk contained penicillin. Bulk milk was again free, but small aggregates of a few supplies used for making bulk starters sometimes contained penicillin.

3. Of twenty samples of milk and seven of whey examined during 1951–3, and which were implicated in starter failure, only two milks and one whey contained penicillin in significant amounts.

4. The rate of excretion of penicillin from the composite milk of a herd containing thirty-seven treated cows was determined. Milk up to the fifth milking contained sufficient penicillin to cause starter inhibition.