Abstract <p>The effect of γ-irradiation of papain with doses from 30 to 700 kGy on the absorption of ultraviolet and visible light, isoelectric state, viscosity, surface tension, and electrical conductivity of its aqueous solutions has been studied. It has been shown that an increase in the papain irradiation dose, on one hand, is accompanied by an increase in UV and visible absorption and an enhancement in conductivity and, on the other hand, leads to a decrease in the viscosity and surface tension of papain solutions. The main reasons for such changes are the destruction of peptide bonds in papain during radiolysis and a decrease in its molecular weight, radiation-induced oxidation of amino acid residues, the formation of carbonyl derivatives, and the formation and accumulation in papain macromolecules of various terminal primary amino groups that carry a positive charge.</p>