In order to describe the electrophoretic behavior of "Hairy-layer"-coated particles, local charge density isotherms have been combined with considerations of hydrodynamic flow penetration into the layer. An iterative numerical procedure was employed to calculate the electrophoretic mobility as a function of both pH and salt concentration. A major result is that information can be gained regarding the distribution functions of oppositely charged groups from the behavior of the point of zero mobility as a function of pH and salt concentration. The theoretical results were used to reinterprete the electrophoretic behavior of native, neuraminidase-treated and glutaraldehyde-treated human erythrocytes. Theoretical analysis of the experimental data revealed that negative groups had occurred preferentially in the outer regions of the layer. A penetration depth of 1 nm into the 3.5 nm thick hairy layer successfully accounted for the erythrocyte mobility data for all methods of analysis tried. Upon sialic acid removal, alterations of the glycocalyx structure occurred, resulting in possible changes in the fixed charge distribution. It was concluded that intramolecular, as well as intermolecular, electrostatic interactions are important for the structure of the hairy surface layer (glycocalyx). No evidence for the adsorption of small ions was found.