Hypotaurine was determined colorimetrically by oxidizing with HIO and converting the excess of HIO into I 2, followed by the iodine-starch reaction. Determination of hypotaurine in the range 5–40 mμmoles/ml was found to be possible by this method. Sulfurous oxide, cysteine, acetone, aldehyde, protein, and organic acids interfered with hypotaurine determination.