Reduction/oxidation (redox) processes have been implicated in various biologic processes, including signal transduction, gene expression, and cell proliferation. Thioredoxin is one of the major redox-regulatory molecules which because of its dithiol/disulfide exchange activity determines the oxidation state of protein thiols. In this study, we report that treatment of several cell types with thioredoxin strongly enhances the expression of various cytokines. In monocytic Mono Mac6 cells stimulated with the phorbolester tetradecanoyl phorbolacetate (TPA), thioredoxin was found to augment the expression of TNF at the protein and mRNA levels. In this and other cell lines, such as fibrosarcoma and endothelial cells, thioredoxin also dose-dependently increased the synthesis of IL-6. Treatment of TPA-stimulated Mono Mac6 cells resulted in a strong potentiation of secreted IL-1 bioactivity and expression of IL-1 alpha and IL-8 mRNA. In addition, in TPA-activated Molt-4 T cells, an increased expression of IL-2 and IL-2-specific transcripts was detected. These data demonstrate that cytokine synthesis may be tightly controlled by redox-dependent processes. As thioredoxin is readily secreted and taken up by cells, it may play an important role as a costimulatory molecule involved in immune processes.