A new colorimetric method has been established to determine ethylestrenol (17α-ethyl-17β-hydroxyestr-4-ene), which lacks a functional group available to known color reactions, and was assayed hitherto by the unspecific method with concentrated sulfuric acid. It gives a stable blue color with the reagents, anhydrous aluminum chloride dissolved in nitrobenzene and anisaldehyde in nitrobenzene-benzene (1 : 1). The assay is carried out in a range of 100∼200μg. of the steroid per 1 ml., and successfully applied also to the determination of its preparation.