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The kinetics of soluble aggregate formation in equine IgG was studied in the pH 3.4-4.3 range and ionic strength between 0.02 and 0.5 M, and a diagram describing aggregation kinetics as diffusion limited, reaction limited, or transitional as a function of pH and ionic strength was constructed. Aggregation rate is limited by the degree of electrostatic repulsion between the protein molecules in the pH 4.0-4.5 range. Below pH 4.0, a greater degree of attractive force is present, most likely from protein unfolding, and electrostatic repulsion no longer determines the rate of aggregation. The aggregation rate increases with decreasing pH, and at pH 3.4 the aggregation rate is diffusion limited. The pH range separating reaction-limited and diffusion-limited kinetics decreases with increasing ionic strength, indicating charge shielding from the buffer solution influences the aggregation rate. Copyright 1997 Academic Press.