The purpose of this study was to determine how oligodendrocytes die following Fas receptor activation. An immortalized human oligodendrocyte hybrid line (MO3.13) was challenged with Fas ligand (FasL), and cell death was assessed by flow cytometry and DNA gel electrophoresis. Caspase activation was determined by either Western immunoblotting on cell extracts or by whole-cell flow cytometry. FasL challenge clearly induced substantial apoptotic cell death in the oligodendrocyte hybrid cell line, as judged by flow cytometry and by the presence of prominent low molecular weight DNA banding patterns after gel electrophoresis. Western immunoblots showed marked increases in cleaved caspase-1, 8, and 3, indicating that the extrinsic Fas death receptor-induced pathway was activated. The intrinsic mitochondrial pathway was also activated, but only at a minimal level. These findings demonstrate that there are several independent molecular sites within the extrinsic caspase cascade in oligodendrocytes where inhibitory compounds may be capable of blocking cell death in vivo.