Although beta-galactosidases are physiologically a very important enzyme and have may therapeutics applications, very little is known about the stability and the folding aspects of the enzyme. We have used beta-galactosidase from Pisum sativum (PsBGAL) as model system to investigate stability, folding, and function relationship of beta-galactosidases. PsBGAL is a vacuolar protein which has a tendency to multimerize at acidic pH with protein concentration >= 100 mu g mL(-1) and dissociates into its subunits above neutral pH. It exhibits maximum activity as well as stability under acidic conditions. Further, it has different conformational orientations and core secondary structures at different pH. Substantial predominance of beta-content and interfacial interactions through Trp residues play crucial role in pH-dependent multimerization of enzyme. Equilibrium unfolding of PsBGAL at acidic pH follows four-state model when monitored by changes in the secondary structure with two intermediates: one resembling to molten globule-like state while unfolding seen from activity and tertiary structure of PsBGAL fits to two-state model. Unfolding of PsBGAL at higher pH always follows two-state model. Furthermore, unfolding of PsBGAL reveals that it has at least two domains: alpha/beta barrel containing catalytic site and the other is rich in beta-content responsible for enzyme multimerization.