An experimental study on the colloidal stability of latex-F(ab')2 and latex-IgG systems is described. The stability domains were obtained using a low-angle scattering technique to measure the rates of aggregate formation and plotting the stability ratio as a function of electrolyte concentration. The protein-coated particles present an anomalous stability at high ionic strength when the classical theory predicts aggregation. This observed deviation from DLVO behavior appears for electrolyte concentrations above some critical bulk concentration called critical stabilization concentration (csc). As we have suggested in previous publications, the existence of an additional short-range repulsive "hydration force" can explain this anomalous stability. In order to resolve more fully if this anomalous stability at high ionic strength is due to the hydration forces, the effects of pH, cation and anion type, temperature, and polyethylene glycol (a dehydrating polymer) on the experimental stability are investigated. Finally, the immunoreactivity of an anti-CRP F(ab')2-latex conjugate is studied in reaction buffers with high ionic strength. Copyright 1998 Academic Press.