Abstract A quantitative determination of free, conjugated, and total glucuronic acid in pure and biological systems (serum and urine) has been developed employing a modified naphthoresorcinol procedure. Glucose interference is eliminated selectively with glucose oxidase which permits the determination of glucuronic acid in the presence of high concentrations of glucose. Free glucuronic acid in decomposed with NaOH to a naphthoresorcinol-negative product in the determination of O-glucuronides. The procedure utilizes a modified naphthoresorcinol reagent and a reduced volume of extracting agent. The method is six to eight times more sensitive than the previous naphthoresorcinol procedures with a molar absorptivity of 30.1 × 10 3. The reagent is stable for 7 days and yields reproducible results. The method of standard addition is employed for the analysis, which results in increased accuracy at low concentrations in the determination of free and conjugated glucuronic acid. The accuracy of the method is ±5% with a precision of ±0.35 μ g 3 ml in the optimum concentration range of 3.5–16.6 μ g 3 ml . The total time required for a complete analysis on either serum or urine is 4 hours.