The subcellular origin of ceramide signaling in ionizing radiation-triggered apoptosis was investigated using two previously described subclones of the autonomous erythro-myeloblastic cell line TF-1, radio-resistant and -sensitive TF-1-34 and TF-1-33, respectively. We show in nuclei-free lysates and cytoplasts that both cell lines failed to generate ceramide in response to ionizing radiation. Moreover, whereas cytoplasts did respond to anti-Fas stimulation through phosphatidylserine externalization, no effect was observed with ionizing radiation. Only in highly purified nuclei preparations did we observe ceramide generation, neutral sphingomyelinase activation, and apoptotic features (PARP cleavage, nuclear fragmentation, DNA laddering) in TF-1-33, but not in TF-1-34 cells. These observations suggest that nuclear sphingomyelinase and ceramide formation may contribute to ionizing radiation-triggered apoptosis.