c-Rel is a member of the Rel/NF-kappaB transcription factor family and is predominantly expressed in lymphoid and myeloid cells, playing a critical role in lymphocyte proliferation and survival. Persistent activation of the c-Rel signal transduction pathway is associated with allergies, inflammation, autoimmune diseases, and a variety of human malignancies. To explore the potential of targeting c-Rel as a therapeutic agent for these disorders, we designed a small interfering RNA (siRNA) to silence c-Rel expression in vitro and in vivo. C-Rel-siRNA expression via a retroviral vector in a B cell tumor cell line leads to growth arrest and apoptosis of the tumor cells. Silencing c-Rel in primary B cells in vitro compromises their proliferative and survival response to CD40 activation signals, similar to the impaired response of c-Rel knockout B cells. Most important, in vivo silencing of c-Rel results in significant impairment in T cell-mediated immune responses to antigenic stimulation. Our study thus validates the efficacy of c-Rel-siRNA, and suggests the development of siRNA-based therapy, as well as small molecular inhibitors for the treatment of B cell tumors as well as autoimmune diseases.