Furfural induces the accumulation of reactive oxygen species (ROS). Exponentially growing yeast cells were treated with no inhibitor, 25 mM furfural, 50 mM furfural or 5 mM hydrogen peroxide (positive control for ROS). (A) Representative images of cells stained with the ROS indicator dye 2'7' DCF-diacetate (left column) and differential interference contrast (DIC) (right column) are shown. Images of cells were at the 8 h time point. (B) Percent of yeast cells that stained positive for ROS by 2'7' DCF-diacetate at 0 h and 8 h. Data represent an average of five experiments with standard error indicated. At each time point at least 100 cells were examined.