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Revised Manuscript Received January 23, 1992 abstract: Free radicals produce a wide spectrum of damages; among these are DNA base damages and abasic (AP) sites. Although several methods have been used to detect and quantify AP sites, they either are relatively laborious or require the use of radioactivity. A novel reagent for detecting abasic sites in DNA was prepared by reacting0-(carboxymethyl) hydroxylamine with biotin hydrazide in the presence of car-bodiimide. This reagent, called Aldehyde Reactive Probe (ARP), specificially tagged AP sites in DNA with biotin residues. The number of biotin-tagged AP sites was then determined colorimetrically by an ELISA-like assay using avidin/biotin complex conjugated to horseradish peroxidase as the indicator enzyme. With heat/acid-depurinated calf thymus or bacteriophage fl DNA, ARP detected femtomoles of AP sites in DNA. Using this assay, DNA …