Disclosed is an agent for promoting the differentiation of an osteoblast andinhibiting the differentiation of an osteoclast, which comprises a milk proteinfraction having the following chracteristic properties (1) to (4): (1) the milkprotein fraction is derived from milk； (2) the milk protein fraction is a fractionthat contains a protein component having a molecular weight of 75,000 to 80,000Daltons； (3) in the constituent amino acid composition of the milk protein fraction,the basic amino acid content is 13 to 15 wt%, and the ratio of the amount of a basicamino acid(s) to the amount of an acidic amino acid(s) [basic amino acid(s)/acidicamino acid(s)] is 0.5 to 0.7； and (4) the milk protein fraction has an activityof promoting the differentiation of an osteoblast and inhibiting the differentiationof an osteoclast.
