Abstract: An approved procedure for the purification and renaturation of biologically active, bacterially produced IFN-.beta. is described. The partially purified material obtained by solubilization of isolated refractile bodies from the recombinant cells is treated to obtain reduction of the protein in the presence of a chaotropic environment and then oxidized after removal of the reducing agent. However, the chaotropic environment is retained during the oxidation. Upon removal of the chaotropic environment, a solubilizing additive is supplied to maintain the IFN-.beta. in solution. Further purification by conventional means may also be effected.