When recombinant human interleukin 2 (rIL-2) was stored for a long period in aqueous solutions, its methionine residues were spontaneously oxidized the corresponding methionine sulfoxides. To clarify the oxidation process, rIL-2 was treated with hydrogen peroxide, and three oxidation products were separated by reversed-phase high-performance liquid chromatography. By means of amino acid analyses after cyanogen bromide cleavage of these products, we found that Met104, Met23, and Met39 were consecutively oxidized in this order; Met46 was not oxidized easily. The substances generated by extended storage of rIL-2 in aqueous solutions were identified by comparison of their peptide maps with those of the chemically oxidized products. The protein-chemical properties of these products, except for the modifications of the methionine residues, were similar to those of intact rIL-2; their biological activities were almost the same.