The active component on the proliferation of osteoblastic MC3T3-E1 cells was purified and identified from bovine milk. The growth-promoting activity was measured by [H-3]thymidine incorporation on the cell. The purified protein showed a molecular size of 17 kDa on SDS-PAGE. Its amino-terminal amino acid sequence was very similar to the internal sequence of bovine high molecular weight (HMW) kininogen, which comprises fragment 1.2. The promotion of proliferation was specific for osteoblastic MC3T3-E1 cells, not for fibroblast BALB/3T3 cells. In blood coagulation, HMW kininogen is considered to be cleaved by a specific enzyme kallikrein. HMW kininogen then releases two peptides, a biologically active peptide bradykinin and fragment 1.2, but the fate of fragment 1.2 is unknown. This milk-derived protein that comprises to fragment 1.2 showed a growth-promoting activity of osteoblasts. We propose the possibility that milk plays an important role in bone formation by supplying the active agent for osteoblasts as well as supplying calcium.