A polypeptide having interferon activity which has an amino acid sequence corresponding to an amino acid sequence of an interferon selected from rIFN-.alpha., hybrid rIFN-.alpha., rIFN-.beta. and rIFN-.gamma. interferons in which at least one cysteine residue in the amino acid sequence has been replaced by an amino acid residue which is incapable of forming intermolecular disulfide bonds is provided. There are also provided a dsDNA sequence encoding the novel polypeptide; a replicable plasmidic expression vehicle containing the dsDNA encoding the novel polypeptides of the invention; a microorganism which has been transformed with the replicable plasmidic expression vehicle; and a method of preparing the dsDNA which encodes the novel polypeptides of the invention.