Cell volume regulation is an intermediate step in the process of apoptosis. Among the regulators of cell volume is the cellular accumulation of organic osmolytes including the b-amino acid taurine. The carrier accomplishing concentrative cellular uptake of taurine is the taurine transporter TAUT. Analysis of taut(/(mice revealed that the numbers of activated CD69+ CD4+ and CD69+ CD8+ T cells were significantly decreased in spleen cells from taut-/- mice. Therefore, lymph node analysis was performed in mice sensitized with trinitrochlorobenzene (TNCB) to elicit contact hypersensitivity. TNCB-sensitized taut(/( mice showed reduced T cell numbers within skin draining lymph nodes indicating a specific role for TAUT in activation-induced cell death of T cells. Indeed, enhanced exposure of phosphatidylserine in CD8+ T cells from taut-/- as compared to taut+/+ mice pointed to enhanced apoptosis of those cells. The present observations point to a functional significance of concentrative taurine uptake for the survival of both CD4+ CD3+ T helper and CD8+ CD3+ cytotoxic T cells, especially after activation.