Cardiocrome (CC) is a cellular respiration activating drug consisting of cytochrome-C (Cyt-C), thiamin diphosphate (TPP) and FMN. CC has been used for various ischemic disorders, such as cerebrovascular disorder and cardiac disease. To elucidate the mechanism of action of CC, we investigated the effects of CC and its components on arsenite-induced apoptosis in U-937 cells, human promonocytic cell line. Arsenite rapidly caused apoptosis in U-937 cells. Treatment of arsenite, at a dose of 80 μM, to U-937 cells for 24h caused DNA fragmentation and induced morphol. alternations, apoptotic bodies, and also significantly increased the rate of apoptosis in U-937 cells to 24.2± 2.7% (mean ± SE, n=8, p< 0.01 vs. control (0.0±0.0%)); On the other hand, treatment of CC, at a dose of 100 μg/mL, inhibited the DNA fragmentation and the induction of apoptotic bodies by arsenite. CC, at a same dose, also significantly decreased the arsenite-induced apoptosis by 11.0±2.1% (p< 0.01); This effect was mainly due to the action of FMN and enhanced by the combination of three components mentioned above, i.e. CC. Furthermore, CC also suppressed heat shock protein 70 expression caused by arsenite. On the contrary, CC did not affect DNA fragmentation by actinomycin-D. These results suggest that CC may be related, at least in part, to relief of the oxidative stress induced by stressor(s) such as arsenite, and consequently protect cells or tissues from apoptotic cell death.