2-Nitro-9-fluorenone is a mutagenic photoproduct of u. v. a. -irradiated 2-aminofluorene. 2-Nitro-9-fluorenone was isolated from diesel-exhaust particles using a two-step fractionation scheme consisting of Sephadex LH20 chromatography and silica-gel thin-layer chromatography. Submicromolar concentrations of 2-nitro-9-fluorenone were quantitated by mercury meniscus modified silver solid amalgam electrode combined with direct current voltammetry or differential pulse voltammetry.