A basic somatomedin preparation results in a 2-fold stimulation of incorporation of both [3H]leucine into protein and 35SO4 into glycosaminoglycans by cultured chick sternal chondrocytes. The stimulation is seen in the presence of 20 mM 4-methylumbelliferyl-beta-D-xyloside alone but is not observable in the presence of both xyloside and cycloheximide, indicating that the stimulation by somatomedin is dependent on protein synthesis. Somatomedin stimulates incorporation of leucine into glycosaminoglycans to the same extent to which it stimulates incorporation of sulfate into glycosaminoglycans, suggesting involvement of core protein synthesis. In the presence of the xyloside, however, there is stimulation of synthesis of glycosaminoglycan chains initiated on the xyloside, with a shift in molecular weight distribution to larger molecular weights. Thus, when core protein is not rate limiting, a more general stimulatory effect of somatomedin is observed.