[en] Iodine-125 was used as labeling nuclide, and the PEG-rhIL-6 was labeled by the common used chloramines-T and the two-phase chloramines-T, respectively. The labeled compound was purified by both methods of gel filtration and ultrafiltration respectively. The purity of the labeled PEG-rhIL-6 was determined by both trichloroacetic acid (TCA) and SDS-PAGE, and the biological activity was determined by MTT method. The results demonstrated that the labeling rate and specific radioactivity were 74.5% and 5.513 x 10⁵ Bq/μg for PEG-rhIL-6 by the two-phase chloramines-T method, higher than that by the common used chloramines-T method, which was 62.3% and 4.610 x 10⁵ Bq/μg respectively. The purity of labeled PEG-rhIL-6, purified by both gel filtration and ultrafiltration methods, was over 99% with TCA method. The labeled PEG-rhIL-6 by two-phase chloramines-T method showed two bands, which was identical to that of standard PEG-rhIL-6 though SDS-PAGE, but the labeled PEG-rhIL-6 by common used chloramines-T method had one more band compared with standard PEG-rhIL-6. When determined by MTT, it shown that the biological activity of PEG-rhIL-6 iodinated by common used chloramines-T method was lower than that by two-phase chloramines-T method. (authors)