Hyper-Rayleigh scattering is used to measure the nonlinear optical properties of collagen in acetic acid solution. We find that the first hyperpolarizability unexpectedly drops with the increase of the concentration of collagen in solution. Circular dichroism absorption measurements indicate that the helical conformation remains stable, and the changes of nonlinear optical properties of collagen molecules are, most probably, due to the formation of some supramolecular structures of collagen in solution. (C) 2004 Elsevier B.V. All rights reserved.