The binding affinities of polyanions for bovine serum albumin in NaCl solutions from I = 0.01-0.6 M, were evaluated on the basis of the pH at the point of incipient binding, converting each such pH, value into a critical protein charge Z(c). Analogous values of critical charge for mixed micelles were obtained as the cationic surfactant mote fraction Y-c. The data were well fitted as Y-c or Z(c) = KIa, and values of K and a were considered as a function of normalized polymer charge densities (T), charge mobility, and chain stiffness. Binding increased with chain flexibility and charge mobility, as expected from simulations and theory. Complex effects of tau were related to intrapolyanion repulsions within micelle-bound loops (seen in the simulations) or negative protein domain-polyanion repulsions. The linearity of Z(c) with root I at I < 0.3 M was explained by using protein electrostatic images, showing that Z(c) at I < 0.3 M depends on a single positive "patch"; the appearance of multiple positive domains I > 0.3 M (lower pH(c)) disrupts this simple behavior.