Electrophoretic mobilities, mu, of nine proteins (M(r) 14,200 to 70,000) in 28 mM Tris/47 mM glycine buffer at pH 8.77 and 5 mM ionic strength were measured by laser Doppler velocimetry and correlated to ratios of charge (q) to molecular weight (M(r)) and shape factor (f/f0) by the equation mu(f/f0) = (Aq/Mpr-B). This correlation was previously reported for peptides and proteins for mu measured at 100 mM ionic strength. When A = 6.048 x 10(-3), B = 1.13 x 10(-5), and p = 2/3, the correlation fitted 51 measured and literature values over the molecular weight range of 178 to 140,000 for components whose electrophoretic mobilities ranged from +13.35 x 10(-5) to -19.7 x 10(-5) cm2/(V.s). The experimental measurements confirm the general suitability of p = 2/3 and show that the familiar charge/mass relation for electrophoresis is applicable to proteins in low-ionic-strength buffers which are typical of electrochromatography systems. Extrapolation of the correlation to different ionic strengths indicates that a low-ionic-strength buffer amplifies differences of electrophoretic mobility as a function of charge/mass, while high ionic strength diminishes such differences.