The present invention provides a stable, isotonic, aqueous solution formulation comprising: (i) 0.1-0.5 mg/mL alpha-type interferon, preferably pegylated alpha-type interferon； (ii) 20 mM Acetate buffer system to maintain a pH of 6.0±0.5； (iii) 5-20 mM L-methionine； (iv) 120-150 mM sodium chloride； (v) 0.01-0.07 percent by weight of a surfactant effective to stabilize the alpha-type interferon, preferably pegylated alpha-type interferon against loss of its activity and (vi) an amount of water for injection sufficient to prepare a solution of the above-listed ingredients.
