Rabbit articular chondrocytes cultured in the presence of insulin-like growth factor I (IGF I) increased significantly the synthesis of both collagen and glycosaminoglycans. The increase in the ratio of35SO 4 2− to [3H]glucosamine observed in glycosaminoglycans synthesized in the presence of IGF I seems to indicate that IGF I affects sulphation and synthesis of these polyanionic macromolecules to a different extent.