OBJECTIVETo explore the protective effect and mechanism of Yixinkang (H303) capsule, a composite preparation of Chinese herbal medicine for antagonizing myocardial ischemia, containing serum on oxygen stress damaged mitochondrial membrane potential (MMP) in guinea-pig myocytes.METHODSMyocyte damage was induced by incubating myocytes with hydroxy free radical (Fe2+/H2O2) generating system for 30-60 mins, then H303-containing serum was added in to observe the protective effect. MMP was detected by laser scanning confocal microscopy.RESULTSAfter oxygen stress, the cytochrome oxidase (CCO) activity and MMP in mitochondria significantly decreased. These changes could be significantly lessened by H303-containing serum, demonstrated as obviously increase of CCO activity and myocyte survival rate. H303-containing serum also showed a significant protective effect on membrane potential of myocytes.CONCLUSIONOxygen stress could cause decrease of MMP in myocytes and H303-containing serum could protect it to maintain the membrane stability of mitochondria.