We have recently revealed that the thymus is the organ showing the highest expression of thromboxane (TX) A2 receptor in mice. In this study, thymic cell populations expressing the receptor were identified, and the effects of a TXA2 agonist on these cells were examined. Radioligand binding using a TXA2 receptor-specific radioligand revealed a single class of binding sites in the thymocytes with an affinity and specificity identical to those reported for the TXA2 receptor. The receptor density in these cells was comparable to that seen in blood platelets. This receptor is most highly expressed in CD4-8- and CD4+8+ immature thymocytes, followed by CD4+8- and CD4-8+ cells. The receptor density in splenic T cells was less than one fifth of that in CD4+8+ cells and no binding activity was detectable in splenic B cells. The addition of a TXA2 agonist, STA2, to thymocytes induced the disappearance of the CD4+8+ cells in a time- and concentration-dependent manner and caused DNA fragmentation. These changes were blocked by a specific TXA2 antagonist, S-145. These results demonstrate that TXA2 induces apoptotic cell death in immature thymocytes by acting on the TXA2 receptor on their cell surface and suggest a role for the TXA2/TXA2 receptor system in the thymic micro-environment.