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Electrophoretic mobilities of polystyrene particles coated with three casein proteins (α sl , β, and κ) have been measured at 25°C in 20 m M imidazole buffer (pH 7.5) as a function of ionic strength of added sodium chloride (0.01–0.15 mole dm −3 ) or calcium chloride (0.00003–0.09 mole dm −3 ), with protein concentrations sufficient to ensure saturation coverage as shown by measured adsorption isotherms. Particle mobilities were found to lie in the order α sl > κ ≳ β in NaCl and at low concentrations of CaCl 2 , but in the order κ > α sl ≳ β at higher concentrations of CaCl 2 , reflecting the different calcium-binding properties of the three proteins. Qualitatively similar results for bare and coated particles show that the large difference between sodium and calcium counterions is not due to specific calcium ion-binding on the adsorbed protein. For these biocolloidal particles, the two techniques of laser Doppler electrophoresis and particle microelectrophoresis give essentially identical results.