Injection of soluble protein antigen into animals causes abortive proliferation of the responding T cells. Immunological adjuvants boost T cell responses at least in part by increasing the survival of activated T cells during and after the initial proliferative phase of their clonal expansion. To understand how adjuvants promote T cell survival, we used gene microarrays to analyze gene expression in T cells activated either with antigen alone or in the presence of two different adjuvants. Among the genes whose expression was increased by both adjuvants was the IkappaB family member Bcl-3. Retroviral infection experiments showed that expression of Bcl-3 increased survival of activated T cells in vitro and in vivo. Adjuvants may therefore improve survival of activated T cells via induction of Bcl-3.