Radioactive Sr and Ca were given to rabbits and rats in fundamental studies on contamination of milk with fission from the fallout. These isotopes were applied singly, injected daily, or allowed to ingest daily ad libitum as contained in drinking water. Transfer of Sr
89 and Ca
45 to the milk was determined by assaying the radioactivities of the rabbit milk which had been milked artificially and the radioactivities of suckling rats. By these methods, the dynamic movement of Sr in the animal body was studied. As a result, it was found that two per cent of the Sr orally ingested had been transferred to the milk. Studies were also made on the effect of full dose of carrier and non-radioactive Ca given at the same time with the isotope. The chemical form of Sr
89 in rabbit milk was investigated by means of alcohol coagulation, determination of isoelectric point, and ultracentrifugation and it was found that Sr
89 was bound to casein in milk. Cow's milk mixed with Sr
89 and Y
90 in vitro was coagulated by rennet and most of the added radioactivity was found in the coagulated fraction. The rate of danger in transference of Sr
90 to the milk product was discussed.
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