BACKGROUND: Lactic acid produced by tumors has been shown to overcome immune surveillance, by suppressing the activation and function of T cells in the tumor microenvironment. The strategies employed to impair tumor cell glycolysis could improve immunosurveillance and tumor growth regulation. Dichloroacetate (DCA) limits the tumor-derived lactic acid by altering the cancer cell metabolism. In this study, the effects of lactic acid on the activation and function of T cells, were analyzed by assessing T cell proliferation, cytokine production and the cellular redox state of T cells. We examined the redox system in T cells by analyzing the intracellular level of reactive oxygen species (ROS), superoxide and glutathione and gene expression of some proteins that have a role in the redox system. Then we co-cultured DCA-treated tumor cells with T cells to examine the effect of reduced tumor-derived lactic acid on proliferative response, cytokine secretion and viability of T cells. RESULT: We found that lactic acid could dampen T cell function through suppression of T cell proliferation and cytokine production as well as restrain the redox system of T cells by decreasing the production of oxidant and antioxidant molecules. DCA decreased the concentration of tumor lactic acid by manipulating glucose metabolism in tumor cells. This led to increases in T cell proliferation and cytokine production and also rescued the T cells from apoptosis. CONCLUSION: Taken together, our results suggest accumulation of lactic acid in the tumor microenvironment restricts T cell responses and could prevent the success of T cell therapy. DCA supports anti-tumor responses of T cells by metabolic reprogramming of tumor cells.