Interleukin-2 (IL-2) is an essential cytokine for T-lymphocyte homeostasis. We have previously reported that all-trans retinoic acid (atRA) enhances the secretion of IL-2 from human peripheral blood T cells in vitro, followed by increased proliferation and inhibition of spontaneous cell death. In this study we used a transgenic IL-2 gene luciferase reporter model to examine the effects of atRA in vivo. In contrast to the observations in human T cells, we found an overall reduction in luciferase-reported IL-2 gene expression in mice treated with atRA. Whole-body luminescence of anti-CD3-treated and non-treated mice was reduced in mice receiving atRA. Accordingly, after 7 hr, IL-2 gene expression was on average 55% lower in the atRA-treated mice compared with the control mice. Furthermore, mice fed a vitamin A-deficient diet had a significantly higher basal level of luciferase activity compared with control mice, demonstrating that vitamin A modulates IL-2 gene expression in vivo. Importantly, the atRA-mediated inhibition of IL-2 gene expression was accompanied by decreased DNA synthesis in murine T cells, suggesting a physiological relevance of the reduced IL-2 gene expression observed in transgenic reporter mice.