An amperometric sensor for the determination of nitric oxide based on its oxidation at +0.90 V vs. SSCE on a polycrystalline platinum electrode modified with Nafion((R)) and cellulose acetate is described. The Nafion((R)) layer serves to eliminate interference effects due to anionically charged species (especially nitrite) by Donnan exclusion while the cellulose acetate film minimizes electrode fouling from non-specific adsorption of proteins and other high mol mass species that are typically found in cellular environments. Since the oxidation of NO on platinum surfaces is known to be strongly dependent on crystallographic orientation with the (111) surface having a very high activity, the use of polycrystalline platinum (which has a relatively high fraction of (111)), was most effective. Modified electrodes could be employed in the determination of NO in the micromolar regime even in the presence of 10 mM nitrite, nitrate and ascorbate without interference. In addition, the presence of bovine serum albumin (at 10 mg/cm(-3)) did not affect the determination. The response of the sensor is fast and reversible. Preliminary experiments with HL-60 promonocytic cell line demonstrated that these electrodes can be employed for in vitro biological studies and point to their feasibility for in vivo applications.