Severe acute respiratory syndrome coronavirus (SARS-CoV) infects many organs, such as lung, liver, and immune organs and causes life-threatening atypical pneumonia, SARS causes high morbidity and mortality rates. The molecular mechanism of SARS pathogenesis remains elusive. Inflammatory stimuli can activate IkappaB kinase (IKK) signalsome and subsequently the nuclear factor kappa B (NF-kappaB), which influences gene expression of cyclooxygenase-2 (Cox-2) along with other transcription factors. In this work, we found that the membrane (M) protein of SARS-CoV physically interacted with IKKbeta using a co-immunoprecipitation assay (IPA). Expression of M suppressed tumor necrosis factor alpha (TNF-alpha) induced NF-kappaB activation using a luciferase reporter assay. Further investigation showed M protein suppressed Cox-2 expression using a luciferase reporter gene assay, RT-PCR and Western blot analysis. The carboxyl terminal of M protein was sufficient for the M protein function. Together, these results indicate that SARS-CoV M suppresses NF-kappaB activity probably through a direct interaction with IKKbeta, resulting in lower Cox-2 expression. Suppression of NF-kappaB activity and Cox-2 expression may contribute to SARS pathogenesis.