Reversed-phase high-performance liquid chromatography (RP-HPLC) and protein-chemical analysis revealed that only Asn⁸⁸ in recombinant human interleukin 2 (rIL-2) is liable to be deamidated during a long period of storage in aqueous solutions (pH 5.0) at 25°C, even though there are eight asparagine and six glutamine residues. The deamidation occurred more easily at 40°C than at 25°C but did not occur at all at tempratures below 5°C. The biological activity of Asn⁸⁸-deamidated rIL-2 was found to be almost the same as that of intact rIL-2, whereas its isoelectric point (pI 7.6) is different from that of intact rIL-2 (pI 7.9).