Disclosed is a method of producing a polysaccharide-polypeptide conjugate by reacting a polysaccharide with a polypeptide which contains at least one free amino group, wherein a polysaccharide carrier comprising vicinal hydroxyl groups is oxidized under ring opening to create vicinal aldehyde groups and is reacted with one or more base-instable antigenic polypeptide(s) containing at least one free amino group, the polypeptide(s) being bound directly to the polysaccharide carrier via at least one azomethine bond.