Fas receptor and tumor necrosis factor receptor-1 (TNFR1) mediate the activation of acid sphingomyelinase (ASMase), which catalyzes the hydrolysis of sphingomyelin to ceramide. Ceramide acts as a second messenger in mediating cell growth, differentiation, stress response, and apoptosis. Ultraviolet (UV) irradiation induces Fas receptor and TNFR1 aggregation. However, the roles of Fas receptor and TNFR1 in mediating UV-induced ASMase activation have not been explored. In this report, we demonstrate that Fas receptor, not TNFR1, mediated UV-induced activation of ASMase. Our data indicate that ASMase activity was not induced with UV irradiation but by TNFalpha in MCF-7 cells that expressed low levels of Fas receptor. In contrast, ASMase was activated by UV irradiation or TNFalpha treatment in Fas stably transfected MCF-7 cells. Immunofluorescence staining of TNFR1 on MCF-7 cells showed that TNFR1 was aggregated after treatment with UV irradiation or TNFalpha. However, UV-induced aggregation of TNFR1 did not lead to induction of ASMase activity. These results suggest that Fas receptor aggregation is solely responsible for UV-induced activation of ASMase. Further, with the use of BJAB and dominant-negative Fas-associated death domain-containing protein (FADD) stably transfected BJAB cells, we demonstrated that dominant-negative FADD partly inhibited UV-induced ASMase activation. Our results suggest that FADD is involved in UV-induced and Fas-mediated signaling pathways for activation of ASMase. Mol. Carcinog. 30:47-55, 2001.