Clonal cell lines of murine osteosarcomas were established and have been maintained in vitro for over a year. By implanting these cultured cells into mice, osteosarcomas, whose histological picutres were exactly the same as those of the original tumors, were easily reproduced. The cultured cells of murine osteosarcomas contain an extremely high level of alkaline phosphatase activity. The cells also secrete a great amount of extracellular alkaline phosphatase in culture media.