Objective To establish an assay for detecting α2-macroglobulin activity in Cohn fraction Ⅳ.Methodsα2-M reacted with trypsin to form α2-M-trypsin complex.After the chromogenic substrate Na-benzoyl-DL-arginine 4-nitroanilide hydrochloride ( BAPNA) was added, absorption at 410 nm was detected with the microplate reader .α2-M activity in Cohn fractionⅣwas quantitatively detected according to the established standard curve of plasma α2-M activity. Result Several critical parameters in this assay were optimized .A standard curve of plasma α2-M activity was established . According to this standard curve ,α2-M activity in Cohn fraction Ⅳsample was detected to be 1.578 PU/ml.Conclusion Using normal human plasma as the reference material , theα2-M activity in Cohn fractionⅣcan be detected through chro-mogenic substrate assay.This study provides a simple method to detect α2-M activity during the purification process of α2-M from Cohn fraction Ⅳ.