Growth cartilage (GC) and resting cartilage (RC) cells from the ribs of young rats were separated and cultured. The cultured GC cells showed remarkable osteogenic potential only with the participation of certain host cells even after cultivation, while RC cells showed no osteogenic activity when transplanted as isografts loaded into Millipore chambers. The GC cells showed marked differences in glycosaminoglycan (GAG) synthesis and alkaline phosphatase activity as compared with RC cells in terms of the effects of various hormones, vitamins, and other agents. A linkage of parathyroid hormone (PTH) and polyamine metabolism was found, and it was suggested that PTH induces successive increase of ornithine decarboxylase activity, polyamine levels, and GAG synthesis in cultured chondrocytes obtained from growing rabbit costochondral junctions. A factor in a family of somatomedins was isolated from the cartilage of fetal calves and called “cartilage-derived factor‘’ (CDF). CDF markedly increased GAG, protein, RNA, and DNA synthesis and cell division of the cultured cartilage cells. The GC cells formed matrix vesicles abundantly in vitro without mineral deposition. Co-culture of GC cells with bone marrow cells resulted in degradation of GAG and then formation of hydroxyapatite crystals in the extracellular matrix. Antirat GC mouse IgG was prepared to label and sort the osteogenic cells in bone marrow by fluorescence-activated cell-sorter II (FACS II). GC antigen-positive and -negative cells after FACS sorting were cultured and examined in terms of proteoglycan synthesis, alkaline phosphatase activity, and matrix vesicle production. The former cell group was found to be very similar to the cultured GC cells.