Peginterferon alfa-2b is a covalent conjugate of recombinant alfa-2b interferon with monomethoxy polyethylene glycol (PEG). The average molecular weight of the PEG portion of the molecule is 12,000 daltons. The average molecular weight of the PEG-Intron molecule is approximately 31,000 daltons. The specific activity of peginterferon alfa-2b is approximately 0.7 × 108 IU/mg protein. Interferon alfa-2b is a water-soluble protein with a molecular weight of 19,271 daltons produced by recombinant DNA techniques. It is obtained from the bacterial fermentation of a strain of Escherichia coli bearing a genetically engineered plasmid containing an interferon gene from human leukocytes. The PEG strand protects the molecule in vivo from proteolytic breakdown, substantially increases its in vivo half-life, and reduces immunogenicity by wrapping around and physically hindering access to the protein portion of the molecule.