Although more effective and potent immunosuppressive agents have recently reduced the incidence of acute rejection, drug-induced toxicity and infection caused by over-immunosuppression occasionally elicit a serious problem. However, no effective assay for evaluating overall patient's immune condition is in widespread use at present. We attempted to measure the stimulation index for mRNA of proliferating cell nuclear antigen (PCNA), which is synthesized in early G1 and S phases of the cell cycle and would be expected to reflect the proliferation capacity of T lymphocytes under the immunosuppressive condition. The stimulation index for PCNA mRNA seemed to be closely related to the immunosuppressive state of renal transplant recipients. Patients with stimulation index less than 2.0 tended to have viral reactivation after transplantation. It was suggested that PCNA mRNA monitoring in peripheral blood could provide a warning of possible over-immunosuppression as one simple assay for immune function monitoring.