Coumarin derivatives have found application as probes for the hydroxyl radical because one of the products of the reaction between them is a highly fluorescent umbelliferone. We have examined the interaction in aqueous solution between a cationic coumarin-labeled hexa-arginine peptide ligand and plasmid DNA, and compared after gamma irradiation the yields of products derived from both of them. At low ionic strengths, the ligand binds very tightly to the plasmid. Compared with the structurally similar 4-methylumbelliferone (phenolic pK(a) = 7.8), the fluorescent product derived from gamma irradiation of the coumarin labeled cationic peptide is significantly more acidic (pK(a) = 6.1), making it a very convenient probe for solutions of pH in the physiological range. The yield of this product is generally in excellent agreement over a wide range of conditions with that of the single strand break product produced by the reaction of the hydroxyl radical with the plasmid. Thus coumarin-labeled peptide ligands offer promise as hydroxyl radical probes for locations in close proximity to DNA.