Ampicillin-sodium and ampicillin-trihydrate were used in the experiments as an active substance at the rate of 800 IU/mg. Ampicillin-sodium was applied in the form of 5 per cent solution, and ampicillin-trihydrate--in the form of a 5 per cent water suspension prepared extempore prior to their administration. Both forms of ampicillin were used once, muscularly, at rates of 0.01 and 0.025 g/kg body weight with calves and sheep. The results of these comparative studies showed that the levels of the serum concentrations of ampicillin and the time of their deposition in both species of animals correlated with the dose, the chemical composition, and the species of animal. The period of retaining the antibiotic in the blood varied in calves and in sheep. Parallel to these studies the excretion of ampicillin from the body was also followed up--in the ewes' milk at muscular administration, rated 0.025 g/kg body weight. It was found that ampicillin-sodium and ampicillin-trihydrate quickly penetrate in the udder, and are observed in the milk at the 30th minute (0.25 microgram--0.38 microgram/cm3).