Electrophoretic mobilities, (\mu), of nine proteins (\left(M_{\mathrm{r}}\right.) 14,200 to 70,000 ) in (28 \mathrm{mM}) Tris/47 (\mathrm{mM}) glycine buffer at (\mathrm{pH} 8.77) and (5 \mathrm{~mm}) ionic strength were measured by laser Doppler velocimetry and correlated to ratios of charge ((q)) to molecular weight (\left(M_{\mathrm{r}}\right)) and shape factor ((f)) (\left.f_{0}\right)) by the equation (\mu\left(f / f_{0}\right)=\left(\mathrm{Aq} / M_{\mathrm{r}}^{p}-\mathrm{B}\right)). This correlation was previously reported for peptides and proteins for (\mu) measured at (100 \mathrm{~mm}) ionic strength. When (\boldsymbol{A}=) (6.048 \times 10^{-3}, B=1.13 \times 10^{-5}), and (p=\frac{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 \times) (10^{-5}) to (-19.7 \times 10^{-5} \mathrm{~cm}^{2} /(\mathrm{V} . \mathrm{s})). The experimental measurements confirm the general suitability of (p=\frac{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. 1995 Academic Press, Inc.