It has been reported that levo-1-methyl tryptophan (L-1MT) can block indoleamine-2,3-dioxygenase (IDO) expressed by human dendritic cells (DC), whereas dextro-1-methyl tryptophan (D-1MT) is inefficient. However, whether L-1MT or D-1MT can efficiently reverse IDO-induced arrest of human T-cell proliferation has not been clarified. Here, we show a marked immunosuppressive effect of IDO derived from INDO-transfected 293 cell, IDO+ ovarian cancer cells, and monocyte-derived DCs on CD4+ Th1 cells, CD8+ T cells, and natural killer cells derived from peripheral blood, ascites, and tumors of ovarian cancer patients. We found that, whereas L-1MT and D/L-1MT can restore proliferation of tumor-derived and peripheral blood T-cell subsets, D-1MT does not effectively restore IDO-induced arrest of T-cell proliferation. Although D-1MT inhibited kynurenine production at high concentrations, L-1MT was more effective in abrogating kynurenine generation and tryptophan depletion, whereas tryptophan was completely depleted by IDO even in the presence of high amounts of D-1MT. Together, the results indicate that, whereas the generation of tryptophan metabolites (kynurenines) by IDO is important in mediating suppression of T-cell proliferation, the degree to which tryptophan depletion is restored by 1MT is also critical in overcoming IDO-induced arrest of T-cell proliferation.