Dichroa febrifuga has been employed by Chinese physicians as a medical cure for malaria since ancient times. Up to the present, no satisfactory quantitative method for the estimation of its effective components, febrifugine and isofebrifugine, is available. The present work presents a colorimetric method based on the principle that o-amino-benzoic acid is produced by the alkaline hydrolysis of the quinazolone structure of the febrifugine or isofebrifugine molecule, which can be determined colorimetrically by cou- pling with N-1-naphthyl ethylene diamine after diazotization. The optical density-concentration curve of the isofebrifugine hydrolysate was found to coincide with that of pure o-aminobenzoic acid and it did follow Beer's Law. An analytical procedure was established after a systematic examination of the conditions of hydrolysis and colour formation. The sensitivity of detection as expressed in the optical density at 530 mμ per microgram of isofebrifugine per millilitre of coloured solution was found to be 0.111/μg/ml.