The roles of cartilage-derived factor and bone-derived growth factor on skeletal growth were evaluated by comparing their effects on cultured chondrocytes with those of multiplication-stimulating activity and mitogenic polypeptides isolated from pituitary glands and a fibrosarcoma. Fibroblast growth factor and DNA synthesis factor derived from a Rhodamine fibrosarcoma increased the incorporation of [ 3H]thymidine into chondrocytes in a dose-dependent manner without either stimulation or inhibition of proteoglycan synthesis. The extent of stimulation of DNA synthesis by fibroblast growth factor and DNA synthesis factor were much greater than those of multiplication-stimulating activity, cartilage-derived factor and bone-derived growth factor. On the contrary, multiplication-stimulating activity, cartilage-derived factor and bone-derived growth factor caused stimulation of proteoglycan synthesis by chondrocytes, measured as incorporation of 35SO 4 2− into material precipitated with cetylpyridinium chloride. This converse relationship between DNA synthesis and proteoglycan synthesis suggests a specific mechanism by which somatomedin and somatomedin-like factors enhance proteoglycan synthesis in chondrocytes. In addition, fibroblast growth factor and DNA synthesis factor had additive effects with multiplication-stimulating activity in increasing incorporation of [ 3H]thymidine, but did not affect multiplication-stimulating activity-induced increase in proteoglycan synthesis. Thus, DNA synthesis factor present in a Rhodamine fibrosarcoma seems to be a fibroblast growth factor-like mitogenic factor rather than a multiplication-stimulating activity-like factor, although a human fibrosarcoma cell line has been shown to produce multiplication-stimulating activity-related peptides (De Larco, J.E. and Todaro G.J. (1978) Nature, 272, 356–358.)