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1.The effect of variations in the kind and amount of protein intake upon the calcium utilization of growing albino rats was studied. Whole milk, whole wheat, casein and gluten, fed singly or in certain combinations, were the sources of protein. Protein was supplied at levels of approximately 9 and 27%. In one of the three experimental periods the amount of calcium present in the diets was approximately one-fourth the amount given in the other two periods. Both ad libitum and restricted feeding methods were employed.2.The final weights of the animals varied considerably, depending upon the amount and kind of protein present in the diet. Animals on the low calcium diets had lighter femurs, containing a smaller percentage of ash and of calcium than did the others.3.A study of the data from animals on restricted feeding indicates that on low levels of calcium intake, casein tends to favor bone formation to a greater extent than does gluten. This may be considered a direct effect of protein on bone formation. Protein may also exert an indirect effect on bone formation by promotion of a growth of soft tissue which is out of proportion to the amount of skeletal tissue. Such an indirect effect of protein was observed in this study when relatively high levels of wheat or gluten, or both, were fed.