Six patients undergoing heart transplantation were followed up by serial endomyocardial biopsies to detect signs of graft rejection and the plasma level of thromboxane B2 was measured at the same time. During the operative procedure and concomitant with histologic signs of acute graft rejection, the plasma level of thromboxane B2 significantly increased. After immunosuppressive treatment with steroids and either antithymocyte globulin or monoclonal antibody, regression of the histologic signs of rejection and a significant fall in the level of thromboxane B2 were documented. We conclude that the plasma level of thromboxane B2 may be useful as an early marker of acute graft rejection after heart transplantation.