Abstract Cyclosporin A (CsA) is the most common immunosuppressive agent used for recipients undergoing allogeneic stem cell transplantation (SCT). CsA require therapeutic drug monitoring (TDM) for both its effectiveness and toxicity, but the schedule of administration and optimal blood levels vary among institutions. To establish better assessment of CsA effect on individual immune responsiveness, we analyzed the proportion of IFN-γ+ or IL-2+ cells among CD4+ lymphocytes using 3-color flow cytometric analysis before and after CsA administration. CsA is administered as 3-hour infusion or oral medication twice daily in equally divided doses. 83 peripheral blood samples from 8 allogeneic BMT recipients are analyzed. Cells were cultured in the presence of phorbol 12-myristate 13-acetate, ionomycin, and brefeldin A at 37°C for 4hrs and then stained for surface markers and intracytoplasmic IFN-γ and IL-2. Blood CsA levels were simultaneously measured. We found that the proportion of IFN-γ+CD4+ or IL-2+CD4+ lymphocytes were inversely proportional to blood CsA levels (Figure 1). To achieve profound inhibition of IL-2 production, high concentration of CsA is required (e.g. 600–800 ng/ml for less that 5 % IL-2+CD4+ cells). In vitro experiments were also conducted using lymphocytes from healthy donors with or without activation with CD3 and CD28 to show how the sensitivity to CsA is different in between activated and resting T cells. We found that higher CsA concentration is required for inhibiting IL-2 production from CD4+ lymphocytes activated with CD3 and CD28 than from resting CD4+ lymphocytes (Figure 2). In this regard, our 3 hour intravenous CsA infusion can be superior to continuous intravenous infusion in inhibiting activated T cells. Further analyses are required, but the simultaneous assessment of cytokine production among CD4+ T cells and blood CsA levels may be a useful index to estimate the degree of immunosuppression afforded by CsA and may enable us individualized therapeutic drug monitoring of this agent. Since more and more patients are undergoing cord blood transplantation (CBT) or reduced-intensity stem cell transplantation (RIST), this assessment may also be useful for the establishment of immunosuppressive therapy for those patients.