C5a is thought to play a role during complement-activated neuronal apoptotic cell death in the central nervous system. The mechanisms responsible are however not well-understood. As mitochondria play a key role during apoptosis, we investigated mitochondria as a potential target for C5a. Using PC12 cells, we demonstrated that exposure to C5a led to inhibition of mitochondrial respiration, dehydrogenase and cytochrome c oxidase activities. Interestingly, an increase in expression of the mitochondrial stress protein chaperonin 60 was also observed, confirming a marked effect of C5a on mitochondrial functions. These observations are the first documented intracellular effects noted for the complement molecule C5a in in-vitro cultured cells.