Detection of oxidative stress in live cells using the Image-iT® LIVE Reactive Oxygen Species (ROS) Kit (I36007). Live bovine pulmonary artery endothelial cells were treated with tert-butyl hydroperoxide to induce oxidative stress (bottom) or were left untreated (top). Cells were then labeled with carboxy-H₂DCFDA, which fluoresces when oxidized by ROS, and nuclei were stained with blue-fluorescent Hoechst 33342. The stressed cells exhibited green fluorescence, signaling an increase in ROS, whereas the untreated cells showed minimal fluorescence.