Abstract: Radioimmunoassay of human plasma ACTH is described. Rabbit was immunized by α 1-28 ACTH coupled with BSA. A specific antiserum was obtained and finaly diluted to 1:32000-60000. The radioiodination of ACTH was processed by chloramine-T method.The radiochemical purity of ¹²⁵I-ACTH was about 90-95.5% and its specific activity about 148-355μci/μg.The sensitivity of the method was 4pg/ml. Assay range was 10-640 pg/ml. The intraassay variation was 2.3-70% while interassay variation 8.5～8.8%.The concentration of human plasma ACTH was determined by this method in 20 health adults and range from 10.5 to 82 pg/ml at 8 a.m., The results of patient were in good accondance with that of clinical diagnosis.