<p>An interferon-beta/polyethylene glycol composite which has high bioactivity; and a process for producing the composite at a high efficiency. The process for interferon-beta composite production is characterized by bonding interferon-beta to polyethylene glycol in the presence of at least one additive selected from the group consisting of oligosaccharides constituted of up to 5 sugar units, monosaccharides, sugar alcohols derived from these saccharides, and C2-6 polyhydric alcohols. The interferon-beta composite produced by the process has polyethylene glycol specifically bonded to the lysine located in the 19th or 134th position in the amino acid sequence of interferon-beta.</p>
