The improtant components of extracellular matrix(ECM) are collagen and chondroitin sulfate. The hydrophobic surface of collagen is one of the determining factors of diameter of collagen fiber and also is closely related to the aging phenomena. The controlling mechanism of the diameter of collagen fiber influenced by the interaction with chondroitin sulfate was evaluated using bis-ANS as a hydrophobic probe. Hydrophobic surface area of collagen molecule shielded by chondroitin sulfate was evaluated. Relative fluorescence intensity of collagen in thepresence of chondroitin sulfate was measured using bis-ANS as a hydrophobic probe. The fluorescence intensity decreased with the increase in chondroitin sulfate up to 3.8 chondroitin sulfate/collagen(mole/mole). Further increase in the ratio of chondroitin sulfate to collagen did not change the fluorescence intensity. Similar changes in the relative fluorescence intensity were observed for both rat tail and lathyrific rat skin collagen. The fluorescence intensity indicated by the binding between bis-ANS and hydrophobic sites of collagen was pH dependent, and the shielding effect of collagen-chondroitin sulfate interaction could not be detected at pH above 6.0. This is probably due to the charge repulsions caused by negative charged collagen molecules at higher pH.