Abstract We have developed a new molecular probe, N,N′-bis(2-hydroxyperoxy-2-methoxyethyl)-1,4,5,8 -naphthalene-tetra-carboxylic-diimide (NP-III), that specifically generates hydroxyl radical upon irradiation with longer wavelength ultraviolet light (UVA). Hydroxyl radicals are generated only upon irradiation, thus NP-III is a new controllable hydroxyl radical source. Apolipoprotein (apo-B) of human low density lipoprotein (LDL), and bovine serum alubumin (BSA), were irradiated with UVA in the presence of NP-III and their oxidation was evaluated by two independent methods: assay of protein carbonyl groups and gel electrophoresis. NP-III oxidized apo-B and BSA in a time- and concentration-dependent manner. The results demonstrate that NP-III is a contollable, precise, and potentially tagetable source of hydroxyl radicals with which to induce protein oxidation.