Coagulation rates of bare and protein-covered colloidal particles show a different dependence on experimental conditions. While the rapid coagulation rate for the bare particles obeys the modified Smoluchowski theory and is independent of pH and the nature of the cation and the anion, the value for the coated particles is lower and depends on pH and ions＇ nature. The variation in the Hamaker constant and the existence of a shallow primary minimum of the interparticle potential for the latex-protein complex, both attributed to the layer of water molecules and ions adsorbed on protein, may explain these results. Coagulation rates were measured with a low angle light scattering apparatus, and the experimental curves of stability fitted using Fuchs＇ equation and the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. In the case of covered particles, a modified expression of the van der Waals attraction was used. This attraction depends on the Hamaker constant for the protein in the vacuum, whose value was estimated from contact angle measurements.