Abstract: A method for the measurement of serum β₂ microglobulin （β₂mG）by radioimmunoassay is presented. β₂mG was isolated from the urine of patients with kidney transplant. ¹²⁵I-labeled β₂mG and rabbit anti-β₂mG antibody were prepared. Polyethylene glycol was used as the separating reagent. The method was simple, precise and sensitive. Serum β₂mG values of normal men （51） and women （40） were 2.18±0.43μg/ml. All 11 patients with renal diseases, 10 of 13 patients with liver diseases, and 19 of 35 patients with hemological diseases had higher values （＞3.1μg/ml）. It appears to be a useful diagnostic procedure for renal, hepatic and malignant hemological diseases.