Process for producing an interferon b complex having a polyethylene glycol specifically and covalently linked to a lysine residue at positions 19 or 134 in the amino acid sequence of interferon bo at the corresponding position thereto in an amino acid sequence of a mutant of interferon b, which comprises: specifically and covalently binding polyethylene glycol, which has a molecular weight of 10,000 or greater and is activated with a functional group reactive with an amino group, to a lysine residue of interferon b according to SEQ ID NO: 1 or an interferon b mutant according to SEQ ID NO: 1 that has a deletion, substitution or addition of one or more amino acids, in the presence, at least, of an additive at a concentration of 1-50% by weight selected from the group comprising oligosaccharides which they have 5 or less sugar units, monosaccharides, their corresponding sugar alcohols and C2-6 polyhydric alcohols, at pH 5.0- 8.5； and extracting, after binding reaction, unreacted interferon b, polyethylene glycol and by-products by using an ion exchange carrier to purify and concentrate the interferon b complex having polyethylene glycol specifically and covalently bound to a lysine residue. at positions 19 or 134 in the amino acid sequence of interferon bo at the corresponding position thereof in an amino acid sequence of a mutant of interferon b, in which the interferon b complex maintains an activity of 10% or higher of the antiviral activity delinterferon b that has not been bound to a polyethylene glycol, when the antiviral activity of interferon b is determined by bioassay using FL cells of human amniocytes and the sydbis virus or the combined stomatitis virus.Procedimiento para producir un complejo de interferón b que tiene un polietilenglicol unido de manera específica ycovalente a un residuo de lisina en las posiciones 19 ó 134 en la secuencia de aminoácidos del interferón b o en lacorrespondiente posición a las mismas en una secuencia de aminoácidos de un mut
