After transplantation, if insufficient immunosuppression is achieved, rejection and graft loss follows. If to much immunosuppression is given, the patient suffers risk for infections and malignancies. Despite careful dosing and monitoring of drug levels, the biological effects of the immunosuppression given is difficult to predict and varies significantly. As a result, the degree of immunosuppression (or immunosuppressive status) remains unknown and clinical problems related to under- or over-immunosuppression are common. Thus, a method to determine the degree of immunosuppression would be of great and direct clinical importance and the results would be improved. T cells are the principal cells of the immunesystem causing rejection. Furthermore, all immunosuppressive regimes targets T cells. Thus, T cell reactivity could reflect the biological effects of the immunosuppression and the immunosuppressive status. In addition, T cells are of crucial importance in the immunedefence against viral diseases. Therefore, data on virus specific T cell reactivity could aid in diagnosis, monitoring and treatment of viral disease. The proposed study aim to develop a clinically useful method to monitor cellular immunity and the degree of immunosuppression after transplantation by determinations of the specific T cell reactivity to several clinically relevant viruses.