AbstractA power series method was applied to solve the Poisson‐Boltzmann equation for the spherical polyelectrolyte model and numerical calculation with an electronic computer was performed to obtain surface electric potential on rigid globular proteins. Deviation from the ideal linear relationship in Linderstrom‐Lang's plot was found to become noticeable as the surface charge density and the radius of protein increases and ionic strength decreases. The calculated surface potential was compared with potentiometric titration data of several proteins whose radii have been analyzed. Assuming the radius of the counterions to be equal to about 1.0 Å, the data for phenolic groups in ribonuclease and for carboxyl groups in conalbumin were interpreted. Reversible intramolecular transformation was found for α‐lactalbumin by comparing the present results with the potentiometric titration data for carboxyl groups. The molecular size of each protein was discussed.