Immunological adjuvants, such as bacterial LPS, increase the mRNA levels of the IkB-related NF-kappaB transcriptional transactivator, Bcl-3, in activated T cells. Adjuvants also increase the life expectancy of activated T cells, as does over-expression of Bcl-3, suggesting that Bcl-3 is part of the pathway whereby adjuvants affect T cell lifespans. However, previous reports, confirmed here, show that adjuvants also increase the life expectancies of Bcl-3-deficient T cells, making Bcl-3's role and effects in adjuvant-induced survival uncertain. To investigate the functions of Bcl-3 further, here we confirm the adjuvant-induced expression of Bcl-3 mRNA and show Bcl-3 induction at the protein level. Bcl-3 was expressed in mice via a transgene driven by the human CD2 promoter. Like other protective events, over-expression of Bcl-3 slows T cell activation very early in T cell responses to antigen, both in vitro and in vivo. This property was intrinsic to the T cells over-expressing the Bcl-3 and did not require Bcl-3 expression by other cells such as antigen-presenting cells.