Cartilage is one of the most commonly manipulated tissue in esthetic and reconstructive surgery. Cartilage has an important role in longitudinal bone growth. Anabolic hormones and locally produced peptide growth factors are known to influence this process Matrix composition changes through proliferation, maturation, and differentiation of chondrocytes, and endochondral ossification thereafter. Defined cartilage matrix is synthesized during the maturation of chondrocytes where the major change is the increment of type II collagen. Variable sulfated mucololysaccharides and hyaluronic acid are also synthesized during this maturation. IGF-I(insulin like growth factor-I), so called somatomedin C, is a prominent growth factor in serum. IGF-I is known to be involved in long growth. IGF-I is affected by pituitary growth hormone. There are few studies done on IGF-I effect in cartilage matrix formation and possible changes of collagen subtypes. This experiment was designed to see the IGF-I effect on the colagen synthesis of cultured chondrocytes. Optimal concentration of IGF-I for the experiment was determined using H3-thymidine incorporation into DNA. The IGF-I effect on collagen synthesis was studied using H3-proline. The IGF-I effect on the synthesis of subtypes of collagen was studied using SDS-PAGE and immunocytochemical staining. Chondrocytes were isolated from the ears of New Zealand white rabbit and cultured in 2 X 10(5) cells/300 microgram density. IGF-I increased DNA synthesis, and optimal concentration of IGF-I was determined by dose-relationship curve as 10ng/ml. Collagen synthesis was increased by IGF-I. Type II collagen was increased on SDS-PAGE with IGF-I and this gel electrophoresis showed type X collagen, also. The increase in type II collagen was confirmed with immunocytochemical staining, the reaction becoming stronger with the addition of IGF-I. Type I collagen was not changed with IGF-I on immunocytochemistry. We conclude that IGE-I is an important modulator influencing not only proliferation and maturation but also terminal different-iation of chondrocytes.