Abstract: Based on the Fenlon's theory the parametric effects in fluids due to nonlinear interaction of a low frequency pump wave and a high frequency probe wave are analysed. The expressions of pressure amplitude of sum or difference frequency sidebands and the formula of non-linearity parameter B/A for lossy medium are derived. The sideband components increase linearly with pump signal. Therefore, a method for determination of B/A is provided and several biological fluids mixtures are measured.