In the present study dibromonitrosobenzene sulfonate (DBNBS) was examined for its suitability for spin trapping for ESR detection of superoxide radicals in biological systems. This nitroso spin trap recently has been reported to yield very persistent spin adducts with O2. as well as with various carbon-centered radicals. In the present work the possible toxicity of DBNBS, the partitioning of its spin adducts into cells, and the stability of the adducts and the parent compound inside cells were studied. No significant toxicity was found. In cellular systems, however, DBNBS did not produce detectable adducts with O2.; it also did not detectably trap superoxide generated in the xanthine/xanthine oxidase system. Both DBNBS and a DBNBS adduct performed extracellularly and then added to cell suspensions were rapidly metabolized by cells. Intracellular spin adducts were not detected under any condition. Evidently, in spite of its promising features, DBNBS will not be useful for spin trapping radicals in cellular systems or for detecting superoxide radicals in any biological system.