Mobility of Skeletal Phosphorus in a Mature Dairy Cow as Determined with Radioactive Phosphorus.

1. Radioactive P32 was injected intravenously to measure the “labile” phosphorus pool in the skeleton of a mature, lac-tating dairy cow. This pool contains about 15% of the total skeletal phosphorus which was 15 times as large as the total circulating phosphorus pool in the plasma. 2. Autoradiograms and radioassays indicated that this labile pool extended throughout the skeleton but its distribution was determined by the type of bone involved. Cortical bone contains only one-third as much phosphorus in the labile state as does trabecular bone. 3. About 12% of the injected phosphorus-32 was present in the skeleton at the time of slaughter of the animal, 3 days after the injection.