AbstractCeramide is recognized as an intracellular mediator of cell growth, differentiation and apoptosis. Tumour necrosis factor, anti‐fas antibody, radiation and anticancer drugs such as actinomycin D are known to induce apoptosis in several cell types through generation of ceramide by activation of the sphingomyelinase pathway or ceramide synthetase. In this study, we examined the occurrence of apoptosis in fibroblasts from patients with Farber disease and from sphingolipid activator protein‐deficient (sap−/−) mouse. These cells accumulate ceramide as the result of genetic deficiency of acid ceramidase and the ceramidase activator (sap‐D), respectively. Amounts of ceramide in fibroblasts from Farber patients and in fibroblasts from sap−/− mouse were increased 2.9‐fold and 2.8‐fold, respectively, over the level of controls. Despite the similar degree of ceramide accumulation, cells exhibiting apoptotic features were increased only in fibroblasts from the sap−/− mouse but not those from the Farber patients. Thymidine uptake of Farber fibroblasts was normal while that of sap−/− mouse fibroblasts was twice normal, consistent with the apparently normal growth and the different rates of apoptotic cell death in these two cell lines. These data suggest that intralysosomal accumulation of ceramide due to defective acid ceramidase or its activator may not play an important role as a mediator of apoptosis. The increased apoptosis in the cultured fibroblasts from the sap−/− mouse may be caused by mechanisms other than the ceramide accumulation. Although more frequent than normal, significant apoptotic cell death was not observed in sap−/− mouse brain in vivo.