ARP (N-(Aminooxyacetyl)-N'-(D-biotinoyl)hydrazine, trifluoroacetic acid salt) reacts with the exposed aldehyde group formed at abasic sites in damaged DNA, allowing the DNA to be labeled with biotin groups. The labeled DNA can then be quantitated with fluorescent or enzyme-conjugated streptavidin complexes. ARP can freely cross the cell membranes, thus allowing detection of abasic sites in living cells. ARP is suitable for use in microplate assays.