An empirical model modified from the Langmuir isotherm model to account for the effects of ionic strength on the equilibrium characteristics of protein adsorption onto ion exchange/affinity adsorbents has been proposed and tested against experimental and literature data. The equilibrium isotherms for BSA adsorption onto a polystyrenic anion exchanger, Diaion HPA25, were established for five different NaCl concentrations at 25 degreesC, pH 7.0. The apparent Langmuir parameters in the new model (q(m)('), and K-d(')), which replace the Langmuir parameters (q(m) and K-d) in the original Langmuir model, were determined by non-linear curve fitting. The proposed model has been shown to be applicable to various protein/adsorbent systems. (C) 2001 Elsevier Science B.V. All rights reserved.