This invention relates to methods and kits for determining the level of H.sub.2O.sub.2 inside a cell, and for determining whether a test compound has ability to scavenge a reactive oxygen species (ROS). The methods and diagnostic kits of this invention employ a cell containing a promoter which is inducible by an ROS, such as the H.sub.2O.sub.2 -inducible KatA promoter of Agrobacterium tumefaciens. The methods of this invention may also be used to select for new or improved ROS scavengers by expressing a library of test scavengers in cells which express a reporter from a ROS-inducible promoter and selecting for those cells whose level of ROS-inducible expression of the reporter is reduced.