Benoxaprofen, a non steroidal antiinflammatory drug is known to be highly phototoxic. Upon irradiation at 300 nm, benoxaprofen is shown to enhance the cleavage of phi X 174 DNA in buffered aqueous solution (pH 7.4). A linear relationship between the number of single strand breaks and the irradiation time is found. In deaerated solutions, these breaks are three times greater in the presence than in the absence of benoxaprofen. In both cases the rate of cleavage decreases in the presence of air. The rate of DNA damage increases with the drug per base pair ratio up to approximatively 0.2 and then decreases at higher ratios. Other NSAIDs, naproxen, ketoprofen, diflunisal, sulindac and indomethacin have been tested as photocleavers of DNA by using the same experimental conditions. A comparison of the efficiency of cleavage of all these drugs (including BNP) was obtained at drug concentrations such that the light absorbance was the same. Benoxaprofen, naproxen, ketoprofen and diflunisal induce single strand breaks. Sulindac and indomethacin do not cause breaks, and they can in some conditions even act as screening agents. The most efficient of the series are naproxen and ketoprofen. In the presence of oxygen, at the same concentrations as above, the efficiency of benoxaprofen, ketoprofen and diflunisal is decreased while that of naproxen is increased. This suggests that all these compounds do not interact with DNA by the same mechanism. In the case of BNP, the mechanism of photoinduced DNA cleavage is discussed in detail. It is shown that the photoactive agent is the decarboxylated derivative of benoxaprofen, as the photodecarboxylation of benoxaprofen is much faster than the photocleavage of DNA.