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The low stability of recombinant human interferon-γ (rhIFN-γ) therapeutic protein imposes some restrictions in its medical applications. In the current study, the effect of oxygen tension on the stability of purified rhIFN-γ was investigated. The rhIFN-γ was purified (>99%) by a two-step chromatographic process. Storage vials were filled by purified formulated product under normal atmospheric oxygen and low oxygen tension conditions. At different time intervals, the amounts of rhIFN-γ covalent dimers and deamidated forms were analyzed using analytical high-performance liquid chromatography (HPLC; size exclusion and cation exchange) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) methods. To determine the biological activity of purified rhIFN-γ, an antiviral activity assay against vesicular stomatitis virus (VSV) was performed. Upon rhIFN-γ long-term storage in a low oxygen tension condition, the amounts of rhIFN-γ covalent dimers and deamidated forms and also the biological activity of rhIFN-γ changed a little. In contrast, by 9 months of storage of rhIFN-γ preparations under normal atmospheric condition, the amount of covalent dimers and deamidated forms increased with time and reached to approximately 3.5% and 11.5% of the initial amount, respectively. The antiviral specific activity of 9-month-old rhIFN-γ preparations decreased to 41% of the initial amount at normal storage condition, while no significant reduction was seen at the low oxygen tension condition. In conclusion, oxygen tension during storage could have a significant impact on rhIFN-γ stability and finally on the quality of pharmaceutical rhIFN-γ product.