The patients after renal transplantation with deteriorating graft function were included to the study. The aim of the study was to establish the usefulness of stabile complement split product C4d expression as the independent indicator of immunological activity in chronic rejection. The C4d expression was studied on fresh frozen tissue with immunofluorescence method. The intensification of immunosuppression regimen was also introduced and azathioprine was exchanged with mycofenolate mofetil. The protocol biopsies a year after inclusion are planned to be done. In our study there was positive correlation between C4d expression and previous acute rejection episodes, morphological changes specific for chronic rejection and clinical improvement after immunosuppression enhancement.