The present invention is directed a method comprising administering a composition to an individual, wherein the composition comprises a plurality of conjugates, each conjugate in the plurality a protein derivatized with a water-soluble polymer, wherein the polymer is coupled to the protein via succinimide groups covalently attached to either cysteine sulfhydryl groups or lysine amino groups, and substantially all of the succinimide groups present in the composition are present in a ring-opened form.