Several acyl-protected hydroxylamines were synthesized as new spin reagents for ESR measurements of intracellular oxidative stress. These compounds were stable non-radical compounds, but were easily deprotected with esterase to yield hydroxylamines, which were oxidized by oxidants to yield ESR- detectable nitroxide radical. Using an acyl-protected hydroxylamine, a highly sensitive ESR determination procedure was successfully conducted to analyze oxidative stress in human leukocytes.