Neopterin (2-amino-4-hydroxy-6-D-erythro-1',2',3'-trihydroxypropylpteridine) is often found in nature (1—3). Recently, the enzymatic formation of neopterin from guanosine triphosphate was demonstrated (4) and the 7,8 dihydro-form of this compound was proved to be an effective precursor for the biosynthesis of folic acid and biopterin (5—7). We wish to report on the isolation in very small amount of a compound whose properties are consistent with a formula of neopterin-3'-β-D-glucuronide (I) from cultures of Azotobacter agilis. The compound was isolated and purified using multiple chromatographic methods including cellulose, DEAE cellulose, and Dowex 1x8 columns, and characterized by elemental analysis, ultraviolet absorption spectra, permanganate oxidation, periodate oxidation, acid hydrolysis, hydrolysis with β-glucuronidase, and paper chromatography and electrophoresis. From these results, analytical data, and results of periodate oxidation the structure neopterin-3'-β-D-glucuronide is proposed for compound I.