Calcium bioavailability to rats was compared from CaCl2 (28 mM), CaCO3, fresh milk, milk adjusted to pH 5.35, and Cheddar cheese. The cheese was manufactured from pasteurized bovine milk and all doses were labeled extrinsically with 45Ca and 47Ca and administered orally to rats. One label (45Ca) was added to milk before cheese manufacture and the other (47Ca) was added to the cheese 24 h prior to dosing. Calcium bioavailability was determined by: 1) absorption measured by whole body counting, and 2) availability for bone metabolism assessed by bone radioactivity measurements. Calcium absorption averaged 76.8% and was not affected by length of ripening (p>0.05). Absorption from CaCl2, CaCO3, fresh milk, milk at pH 5.35, and the cheeses was similar. The two methods gave similar estimates of relative bioavailability. The ratio of 47Ca absorption to 45Ca absorption for any cheese sample was significantly greater than 1, indicating extrinsic labels added after processing may overestimate Ca absorption from cheese.