Aconitase is an iron-sulfur protein containing a [Fe4S4]2+ cluster that catalyzes the stereospecific isomerization of citrate to isocitrate via cis-aconitate. Whereas exposure of aconitase to oxidants renders the enzyme inactive, loss of aconitase activity in cells or in biological samples treated with pro-oxidants has been interpreted as a measure of oxidative damage. Cayman’s Aconitase Assay provides a simple, reproducible, and sensitive tool for assaying aconitase from tissue homogenates or cell lysates.