We report a theoretical and experimental study to reveal the linear relationship between the square of the peak position of the longitudinal plasmon mode of gold nanorods and the square of medium refractive indices. It is predicted theoretically and confirmed experimentally. The linear relationship indicates that gold nanorods can potentially be exploited as a new optical platform to develop novel, specific, sensitive and miniaturized biological sensors after suitable surface modification. (c) 2005 Elsevier B.V.. All rights reserved.