Maps of the surface distribution of the electron donor-acceptor (Lewis acid-base, AB) potential of proteins have been created by (i) identifying the surface residues, (ii) classifying them according to their AB characteristics, and (iii) projecting them onto a cylinder which is then cut and flattened. Net electron donor or acceptor potential densities are determined by an area-preserving gridding procedure, and the variation of these densities with grid size was investigated. Electron donors and accepters are distributed at random on the surface of urease and cytochrome b(5), but not on human serum albumin. The results suggest that the characteristic length of the AB interaction which determines protein adsorption is greater than that implied by macroscopic surface tension measurements and, hence, that interfacial energies derived therefrom overestimate the hydration repulsion between a protein and a surface.