Naphthoresorcinol picrate, 1 : 1 molecular compound, is recommended for the microdetermination of glucuronic acid. The new reagent is advantageous over the usual naphthoresorcinol in the stability on storage, both in crystalline and solution state, and it exhibits higher absorbancy and better reproducibility. The standard procedure was established. It has been found that the measurement of glucuronide with this reagent in the presence of glucuronic acid is possible by employing an alkaline bromine method instead of hypoiodide procedure described by Fishman, et al.