Disclosed is an osteoblast differentiation promoting and osteoclastdifferentiation inhibiting agent which comprises a milk protein fractionhaving thefollowing characteristics (1) to (4): (1) the milk protein fraction is derivedfrom milk； (2)the milk protein fraction is a fraction containing a protein component havinga molecularweight of 75,000 to 85,000 Daltons； (3) the milk protein fraction has a basicamino acidcontent of 13 to 15 wt% in the constitutional amino acid composition, and hasa basicamino acid/acidic amino acid of 0.5 to 0.7； and (4) the milk protein fractionhas anosteoblast differentiation promoting effect and an osteoclast differentiationinhibiting.
