A DNA lesion which results from the breakdown of a pyrimidine base leaving a formamido remnant has been associated with oxidative stress. This lesion is shown to be produced in keratinocytes irradiated in culture with UVB light. The amount of formamido lesion produced is comparable to the amount of the 8-hydroxyguanine lesion. The two lesions were measured by 32P-postlabeling and electrochemical detection methods respectively.