We have modelled the nucleation of the orthorhombic crystal form of lysozyme and subsequent crystal growth from concentration profiles established during the measurement of equilibrium in concentrated solutions of ammonium sulphate. A BCF mechanism for the crystal growth has been assumed. The second osmotic virial coefficient is used to calculate the activity coefficient of the protein in solution. A steep decrease of B-22 is predicted for salt molalities higher than 6.8 m. (C) 2008 Elsevier B.V. All rights reserved.