In the present study, we investigated the dynamic alterations in mitochondrial lipids occurring during Fas- and radiation-induced cell death. Cross-linking of CD-95 on Fas-sensitive Jurkat cells produced rapid increases in two species of mitochondrial phosphatidylglycerol. By 2.5 h, phosphatidylglycerol decreases below basal levels, concomitant with an increase in mitochondrial ceramide. In addition, between 1.5 and 3.0 h after anti-Fas crosslinking, there is a continued loss of mitochondrial cardiolipin. When gamma irradiation was used to induce apoptosis, similar lipid changes occurred, although with somewhat slower kinetics. Fas-resistant Jurkat cells exhibited phosphatidylglycerol as the dominant lipid species in their mitochondria. Following Fas ligation, there is a transient decrease in phosphatidylglycerol, but cardiolipin and ceramide remained unchanged. The high basal levels of PG in Fas-resistant cells and the increase in PG levels in Fas-sensitive cells undergoing apoptosis was determined to be due to increased PGP synthase activity. Thus, critical mitochondrial lipids could potentially serve as novel targets in regulating the apoptotic process.