Abstract: Mesenchymal stem cells (MSCs) are multipotent cells and can be induced in vitro and in vivo to differentiate not only into the variety of mesodermal cells, but into either ectodermal or endodermal cells. This capability indicates the usefulness of MSCs for tissue engineering. Cell surface antigen analyses using various types of CD antibodies demonstrated that adherent fibroblastic cells derived from fresh human bone marrow are mesenchymal types and the cells showed extensive capability for proliferation and/or differentiation. We labeled the adherent cultured marrow cells as MSCs and, significantly, found the MSCs could even proliferate from aged marrow cells. After about sixteen days of culturing, we were able to harvest 100 million MSCs from only 3 ml of fresh human marrow. Moreover, the MSCs could be cryopreserved at −80°C without noticeable loss of viability and capability of osteoblastic differentiation. These results indicate that MSCs hold promise for utilization in hard tissue regeneration.