A theoretical approach to predict the solubility of proteins in solutions containing nonionic polymers is presented. The effective protein-protein interaction due to the presence of the polymer is related to the volume-exclusion potential of Asakura and Oosawa. Statistical-mechanical perturbation theory, as originally applied by Gast et al. to model colloidal flocculation, is used to calculate free energies, from which solubility curves for varying protein-polymer diameter ratios are obtained. The theory correctly predicts all the trends observed in experimental studies of these systems. To explain the influence of processes parameters such as the pH and the ionic strength on protein solubility, the intermolecular potential is improved by the addition of an electrostatic interaction term. It is found that theoretical predictions of the variation in protein solubility, both with the solution pH and the ionic strength, are in accordance with experimental observations.