A new assay for superoxide radicals is based on the interaction of hydroxylamine (1-oxy-2,2,6,6-tetramethyl-4-oxopiperidine) with superoxide, giving rise to a stable nitroxide radical. Working concentration ranges of hydroxylamine and cells are determined. It was shown that the amount of superoxide generated was proportional to the concentration of nitroxide radicals. The sensitivity and specificity of the proposed assay were compared to chemiluminescence and cytochrome-c reduction.