1. In the present study, we examined whether the nuclear transcription factor (NF)-kappaB activity plays a role in the determination of sensitivity to tumour necrosis factor (TNF)-alpha or agonistic Fas antibody (Ab) in human vascular smooth muscle cells (hVSMC). 2. To inhibit agonist-induced NF-kappaB activation in hVSMC, a cell-permeable peptide (SN50), which carried the nuclear localization sequence of the NF-kappaB p50 subunit, was used. Nuclear factor-kappaB activity was examined by both immunoblot analysis of nuclear extracts and by ELISA. The hVSMC were treated with TNF-alpha or agonistic Fas Ab (CH11) and then apoptosis was determined by cell death ELISA for DNA fragmentation. To investigate the mechanisms for protection against apoptosis in hVSMC, we analysed the expression of a conserved family of inhibitor of apoptosis 1 (c-IAP1) protein using immunoblot analysis. 3. Although both CH11 and TNF-alpha alone failed to induce hVSMC death in the presence of SN50, they markedly increased the apoptotic hVSMC estimated by cell death ELISA. In addition, these effects could be blocked with the pan-caspase inhibitor z-VAD.fmk. Western blotting analysis indicated that TNF-alpha alone increased c-IAP1 protein levels, whereas CH11 alone had no effect. Inhibition of NF-kappaB activation by SN50 suppressed c-IAP1 protein expression and enhanced apoptosis induced by either TNF-alpha or CH11. 4. These findings suggest that c-IAP1 is an important intracellular modulator of Fas as well as TNF-alpha death signalling pathways in hVSMC. The expression of c-IAP1 is regulated by a NF-kappaB-mediated phenomenon.