The effects of pH, ionic strength, and polymer dosage level and manner of addition on the fractionation of egg white proteins by precipitation with carboxymethyl cellulose was examined. Using model solutions of egg white proteins, the behavior was quantified through a mechanistic model that accounted for both solubility and aggregation phenomena;A two stage mechanism for the formation of the protein-polymer floc has been proposed, encompassing both formation of the insoluble complex, and aggregation of these complexes into larger particles. The dual role of the polyelectrolyte in the precipitation process, precipitant and flocculant, has been identified;Only proteins of net positive charge were precipitated by the anionic polyelectrolyte and proteins of higher net charge were preferentially precipitated over those of lower net charge. Increased ionic strength levels served to reduce the maximum protein recoveries possible, increase the polymer dosage requirement, and reduce the effect of protein charge on precipitation. At lower polymer dosage levels, protein recovery increases with dosage to an optimum, then decreases with further addition of polymer. This resolubilization effect is reduced by high ionic strength levels, or the use of highly ionized polymers;The complex formation has been modeled as a multi-equilibrium binding process. The binding is the result of electrostatic interactions between the oppositely-charged protein and polyelectrolyte and is negatively cooperative. The ionic strength and charge dependence of the binding constant has been modeled using a modified Debye-Huckel expression. The resolubilization of the protein-polymer complexes is seen to occur at high net complex charge. This residual charge hinders aggregation of the complexes to form insoluble primary particles, resulting in lower protein recoveries.