Complex III (CoQ cytochrome c oxidoreductase) is an essential protein for mitochondrial oxidative phosphorylation.; It accepts electrons from complexes I and II in the form of QH2, the reduced form of the electron carrier ubiquinone.; Once bound to complex III, QH2 undergoes a series of redox reactions, known as the Q-cycle.; During the Q-cycle, electrons are passed from QH2 onto cytochrome c via the Rieske iron-sulfur protein and cytochrome c1 resulting in the translocation of 4H+ and the generation of 02•-.; Cayman's MitoCheck Complex II/III Activity Assay measures the reduction of excess cytochrome c (550 nm absorbance) as catalyzed by complex III.; This assay is coupled to succinate co-enzyme Q oxidoreductase (complex II) for the generation of QH2.; Due to the dependence on complex II activity, a counter-screen for complex II activity (Item No.; 700940) should be performed in order to truly measure complex III activity.