The binding of beta-lactoglobulin to the synthetic polyanion, sodium polystyrenesulfonate, was studied by frontal analysis continuous capillary electrophoresis (Gao et al. Anal. Chem. 1997, 69, 2945). The data were fit to a modified Scatchard plot, and the intrinsic binding constant, K-obs, was measured as a function of pH at fixed ionic strength of 0.05 M. The pH dependence of K-obs was found to follow the semi-logarithmic dependence of Kobs on protein charge Z predicted by Lohman and Record, despite the fact that the net protein charge was of the same sign as the polyanion. However, the magnitude of a log K-obs/partial derivative Z did not agree with the predicted value, either for this system or for pentalysine/DNA data. The current results suggest that the free energy of binding of a protein to a synthetic polyelectrolyte depends on some local protein charge that may vary linearly with the net protein charge.