Mitochondria and Fas (CD95) play a role in tumorigenicity and apoptosis. In the present study, the functional relationship of mitochondria to Fas in mediating apoptosis was investigated. Glioblastoma cells (DBTRGO5MG, U87) were depleted of mitochondrial DNA (mtDNA) by treatment with ethidium bromide (Rho(-) cells). Compared to Rho(+) cells, Rho(-) cells showed enhanced expression of Fas at the cell surface. Indeed, when Rho(+) cells were treated with mitochondrial respiratory chain complex inhibitors, Fas cell surface expression was noted to increase in a similar fashion to the depletion of mtDNA in both cell lines. However, when cells were evaluated for sensitivity to apoptosis using Fas-engagement, there was no difference between the Rho(+) and Rho(-) cells in either cell line. By contrast, sensitivity to the cytotoxic agent cis-diammine-dichloroplatinum (cisplatin) was markedly increased in the Rho(-) cells, which expressed higher levels of cell surface Fas. Expression of Fas is increased with the depletion of mtDNA and respiratory complex inhibitors. However, this increase in expression does not necessarily translate to an increase in sensitivity to Fas-engagement, although there is an increase in the sensitivity of depleted cells to cytotoxic agents such as cisplatin.