The aim of this study is to build and test an amperometric biosensor to determine oxidized glutathione (GSSG). The biosensor is based on glutathione reductase (GR), which was immobilized onto the surface of reduced graphene oxide (rGO) modified with cobalt sulfide (CoS2). CoS2 nanostructures on the composite paper surface are active centers for immobilizing the GR enzyme. The conversion of GSSG into reduced glutathione was monitored with voltammetry in the presence of nicotinamide adenine dinucleotide phosphate reduced form (NADPH) on the GR-CoS2/rGO immobilized electrode surface. The biosensor's cyclic volt response is linearly correlated with GSSG concentrations of 0.01-500 mu M in a 0.1 M pH 7.4 phosphate-buffer solution (PBS). The performance of the sensor was high, with a detection limit of 0.48 mu M and a sensitivity of 0.6 mA mu M cm(-2). Four human blood samples were tested for GSSG with both the biosensor and Ellman's assay, which showed nearly identical results, confirming the biosensor's reliability.