Background: Whereas in animal models the induction of tolerance has been demonstrated to be readily achievable, successful application of such strategies into clinical practice has proved challenging, conceivably due to the difficulty of inducing tolerance in the presence of alloreactive memory T cells. Identification of immunological markers that correlate with good long-term graft function in the presence of minimal immunosuppression would be useful for monitoring allograft outcome and understanding the molecular mechanisms involved in allograft acceptance and establishment of immunological tolerance. In this study we attempted to investigate the balance between effector and regulatory components of the immune system in the early stage and late after kidney transplantation. Methods: We studied subpopulations of circulating memory and regulatory T and B cells in kidney transplant recipients who had good allograft function (creatinine < 1.5 mg/dl) more than 10 years after transplantation under low-dose immunosuppression (LTS, n=11) as well as in patients with good short-term (1-5 years posttransplantation) allograft function on normal triple immunosuppression (STS, n=15). Dialysis patients (n=5) and matched healthy blood donors (n=8) served as additional controls. Circulating T and B cells as well as frequencies of regulatory B and effector and memory T cell subpopulations were measured by flow cytometry. Moreover, the ratio between regulatory and effector or effector memory cells was calculated. Results: While the analysis of the regulatory B subpopulation CD19+CD1d+CD5+ showed no differences between the four groups, LTS patients exhibited a strikingly higher percentage of CD19+CD24highCD38highCD27+ cells (a subpopulation of transitional B cells, TrBCD27+) than the other three study groups (p< 0.001 for all 3 comparisons). Conversely, as compared to STS patients, LTS patients exhibited a decreased percentage of effector (EF, CD27-CD45RO-) and effector memory (EM, CD27-CD45RO+) CD4+ as well as CD8+ T cells. The ratios between TrBCD27+ cells and EF and EM CD4+ and CD8+ T cells were strikingly higher in LTS patients than in STS patients (median values LTS versus STS: 117 versus 18; 14 versus 2, 7 versus 2, and 8 versus 2; p< 0.01, p< 0.001, p< 0.001 and p< 0.001 respectively). Conclusion: Our results indicate that in patients with stable long-term allograft function, an increased percentage of CD27+ transitional regulatory B cells and a decreased percentage of effector and effector memory T cells are involved in long-term allograft acceptance. The strikingly high regulatory B/EF-EM T cell ratio in patients with long-term stable allograft function under low immunosuppression appears to indicate a suppressed alloimmune state and might serve as a clinically useful biomarker for the identification of patients who have developed or are prone to develop operational tolerance and thus represent candidates for withdrawal of immunosuppression.