This chapter describes a protocol for testing nanoparticle formulations for reactive oxygen species generation in male Sprague-Dawley (SD) primary hepatocytes. The protocol utilizes the fluorescent redox active probe, dichlorofluorescein diacetate (DCFH-DA). Primary hepatocytes were chosen for this assay since they have greater metabolic activity than hepatocyte cell lines. This method extends previous standardized cytotoxicity methods for particulates by evaluating mechanisms of toxicity in potential target organ cells. Oxidative stress has been identified as a likely mechanism of nanoparticle toxicity, and cell-based in vitro systems for evaluation of nanoparticle-induced oxidative stress are widely considered an important component of biocompatibility screens.