Simian virus 5 (SV5) is a member of the paramyxovirus family, which includes emerging viruses such as Hendra virus and Nipah virus as well as many important human and animal pathogens that have been known for years. SV5 encodes eight known viral proteins, including a small hydrophobic integral membrane protein (SH) of 44 amino acids. SV5 without the SH gene (rSV5⌬SH) is viable, and growth of rSV5⌬SH in tissue culture cells and viral protein and mRNA production in rSV5⌬SH-infected cells are indistinguishable from those of the wild-type SV5 virus. However, rSV5⌬SH causes increased cytopathic effect (CPE) and apoptosis in MDBK cells and is attenuated in vivo, suggesting the SH protein plays an important role in SV5 pathogenesis. How rSV5⌬SH induces apoptosis in infected cells has been examined in this report. Tumor necrosis factor alpha (TNF-␣), a proinflammatory cytokine, was detected in culture media of rSV5⌬SH-infected cells. Apoptosis induced by rSV5⌬SH was inhibited by neutralizing antibodies against TNF-␣ and TNF-␣ receptor 1 (TNF-R1), suggesting that TNF-␣ played an essential role in rSV5⌬SH-induced apoptosis in a TNF-R1-dependent manner. Examination of important proteins in the TNF-␣ signaling pathway showed that p65, a major NF-B subunit whose activation can lead to transcription of TNF-␣, was first translocated to the nucleus and was capable of binding to DNA and then was targeted for degradation in rSV5⌬SH-infected cells while expression levels of TNF-R1 remained relatively constant. Thus, rSV5⌬SH induced cell death by activating TNF-␣ expression, possibly through activation of the NF-B subunit p65 and then targeting p65 for degradation, leading to apoptosis.