Relatively little is known about how gold nanoparticles (GNP) might interact in vivo with aquatic organisms. Gold nanoparticles (GNP) of defined average diameter may be synthesized and used to challenge test organisms held in aquaria. The blue mussel, Mytilus edulis, is a popular sentinel species in environmental toxicology. This chapter describes how mussels exposed to GNP (average diameter ~5 nm) may be dissected and extracts of digestive gland can be analyzed for oxidative stress. Protein thiols are labeled with 5'-iodoacetamide-fluorescein (IAF), and proteins are separated in one-dimensional electrophoresis. After scanning for IAF-associated fluorescence, gels are stained with colloidal coomassie. A ratio of fluorescence to protein stain is calculated revealing that thiol oxidation increases with GNP treatment.