The performance of an enzymatic colorimetric method for the determination of inorganic phosphorus in serum and urine is described. Phosphate ions react with inosine in the presence of purine nucleoside phosphorylase to form hypoxanthine; this is oxidized by xanthine oxidase to uric acid with production of hydrogen peroxide. The latter is determined with the aid of the chromogen system peroxidase/4-aminophenazone/N-ethyl-N-(3-methylphenyl)-N'-acetylethyl enediamine , the coloured product being measured at 555 nm. This series of reactions is completed in 5 min at 37 degrees C. The test is linear up to 240 mg/l. Analytical recovery in serum averaged 101.2 +/- 1.2% and in urine 101.9 +/- 3.2%. Within-run and between-run precision studies in serum and urine samples gave CVs less than or equal to 4.54% (at 22.0 mg/l). Results obtained by this method agree (r = greater than or equal to 0.983) with the molybdate UV and molybdenum blue methods. Interference by endogenous substances, including organic phosphate, was negligible.