Rabbits have commonly been used for testing the blood-level duration limit of penicillin, but have proved not to be entirely suitable. Experiments reported in this paper on the use of sheep suggest that these animals can be successfully used for the testing of procaine benzylpenicillin with aluminium monostearate (PAM).Penicillin levels were examined in the blood of 79 sheep 72 hours after intramuscular injection of 75 000 International Units of penicillin; 73 of the sheep sera showed a penicillin level corresponding to not less than 0.03 IU of penicillin per ml. The results are also reported of the comparative testing of four batches of penicillin for blood-level duration limits in both human subjects and sheep.