Tumour necrosis factor-alpha (TNF alpha) is a cytokine that induces apoptosis in various cell systems by binding to a TNF receptor (TNFR). To study TNF alpha-induced apoptosis, we isolated and characterized a novel TNF alpha resistant variant, U937/TNF clone II-5, from human monocytic leukaemia U937 cells. The II-5 cells resist apoptosis by TNF alpha and anti-Fas antibody but not by anticancer drugs, such as VP-16 and Ara-C. Somatic cell hybridization between U937 and II-5 showed that the apoptosis resistance to TNF alpha in II-5 was genetically dominant. This dominant mutation in II-5 cells blocks TNF alpha-induced disruption of mitochondrial membrane potential and caspase-3 activation. Expression of TNFR, Fas and Bcl-2 family proteins were not changed in II-5 cells. These results suggest that the apoptosis-resistant II-5 cells could have a functional defect in apoptosis signalling from TNFR to mitochondria and caspase activation. The II-5 cells could be useful in studying the signalling linkage between TNFR and mitochondria.