Abstract The pH dependence of the activity, aggregation, and secondary structure of Artocarpus hirsuta lectin was studied using intrinsic and extrinsic fluorescence, light scattering, and circular dichroism. The lectin is more stable in the neutral and acidic than in the alkaline pH range, which is also reflected in the binding constants of the lectin to methyl α-galactopyranoside (me α-gal). The aggregation of the protein due to heat denaturation is prevented at both extremes of pH. The binding of hydrophobic dye to the lectin takes place at pH 1–2, which increases with increasing temperature. The exposure of hydrophobic patches at pH 1 is reversible. The secondary structure of the lectin is intact in the pH range of 1–8 and is distorted above pH 9. Aggregation of the protein due to heat denaturation is also prevented in the presence of guanidine hydrochloride (GdnHCl).