RIL-2 was labelled with 125I by Iodogen method.The products were punned by HPLC.Analysis of each radiolabelled preparation showed that greater than 95% of the radioiodine was associated with a single protein peak.The specific activity of radioiodinated rIL- 2 was approximately 2.2×1016-2.8×1016Bq/mol. The 125I-rIL-2 was designed for quantitating human IL-2 with sensitive radioimmunoassay (RIA).The bioactivity of 125I- rIL- 2 is good.