The consequences of T‐cell receptor engagement (signal I) are profoundly affected by the presence or absence of co‐stimulation (signal 2). T‐cell receptor (TCR) stimulation in the absence of CD 28‐mediated co‐stimulation not only results in little interleukin (IL)‐2 production, but induces a long lasting hyporesponsive state known as T‐cell clonal anergy The addition of CD 28 ligation to signal 1, on the other hand, results in the production of copious amounts of IL‐2. Our laboratory has utilized CD4⁺ Th1 clones in an effort to understand the molecular events resulting in enhanced IL‐2 production by co‐stimulation and the inhibition of IL‐ 2 production in anergy Our current studies have focused on defining the post‐transcriptional effects of CD28‐enhanced IL‐2 production The data suggest that a major component of CD28′s ability to regulate IL‐2 production occurs at the level of message stability and involves the 3′‐untranslated region of the message. In terms of anergy, our recent studies support the notion that it is not the result of TCR engagement in the absence of co‐stimulation. but rather signal 1 in the absence of IL‐2 receptor signaling and proliferation. Furthermore, T‐cell anergy appears to be an active negative state in which IL‐2 production is inhibited both at the level of signal transduction and by cis‐dominant repression at the level of the IL‐2 promoter.