In T cells stimulated through the T cell receptor (TCR), both cyclosporine (CsA) and glucocorticoids (GC) inhibit the transcription of the IL-2 gene. In these cells costimulation via the CD28 cell surface molecule further increases the transcription of IL-2 and stabilizes its mRNA, resulting in a 20-30 fold induction in IL-2 production. This pathway is relatively resistant to the inhibitory effect of CsA. In this study, we asked whether GC interfere with CD28-mediated costimulatory signals for T cell activation. Primary human T cells or Jurkat T cells were stimulated with anti-CD28 and phorbol myristate acetate (PMA) in the presence of dexamethasone (Dex, 10-10-10-5 M). Dex inhibited both the mRNA for IL-2 and IL-2 production in a dose-dependent fashion (minimum effective dose 10-9 M). In similar experiments employing anti-CD3 mAb and PMA, a 7-20 fold higher concentration of Dex was required to obtain comparable inhibition. To determine if transcriptional modulation is occurring, Jurkat T cells were transfected with a plasmid containing the IL-2 promoter linked to the chloramphenicol acetyl transferase reporter gene. Following stimulation with ionomycin and PMA, high doses (10-6 M) of Dex inhibited the activity of the IL-2 promoter (≈ 50% inhibition). However, in the presence of anti-CD28 mAb, this promoter became resistant to Dex (≤ 10% inhibition). These results suggest that GC inhibit accessory pathways for IL-2 production via CD28 by predominantly posttranscriptional mechanisms. Inhibition of the CD28 pathway may be an important mechanism for the T cell directed immunosuppressive effects of low-to-moderate doses of GC.