Penicillin levels in milk were determined following the intramuscular administration of two types of procaine penicillin G, the aqueous suspension and the oil base with aluminum monostearate. Three injections of the aqueous suspension type were administered, at intervals of 24 hours, to each of twelve cows. A single injection of the oil base type was given to each of twelve cows. In all instances the dose administered was at the approximate level of 5000 u/lb. of body weight. Higher concentrations of penicillin were found in the milk from cows receiving the aqueous suspension type. The highest levels for the two types were 0.52 and 0.15 units per ml. respectively. However, penicillin persisted longer in the milk from cows to which the oil base type was administered. Following the use of each type of procaine penicillin G the levels of this drug in the milk from the different cows of the same series varied considerably. The results of this study indicate a withholding period of at least 60 hours following the last intramuscular injection of procaine penicillin G, aqueous suspension and 108 hours after procaine penicillin G, oil base with aluminum stearate.