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The inhibition of the enzyme indoleamine-2,3-dioxygenase (IDO), that catalyzes the oxidation of the amino acid tryptophan to kynurenine (KYN), is considered a good target for immunoadjuvants in antineoplastic therapy. 1-Methyl-tryptophan (1-MT) is the most studied molecule for this purpose. Although L-1-MT is better than D-1-MT in inhibiting IDO, for an unknown reason the D-enantiomer has higher clinical efficacy. Here we took advantage of co-cultures of tumor cells (SK-Mel 19 melanoma line; 1×105 cells/well) with peripheral blood mononuclear cells (PBMC; 5×106 cells/well) to verify the effect of 1-MT enantiomers on cytokine production and tumoricidal activity. At a concentration that did not affect KYN production, 1-MT (50 µM) affected the production of TNF-α, IL-10, and IFN-γ measured in co-cultures supernatants. Stereospecificity was only observed for IFN-γ production. D-1-MT inhibited more than 30% of IFN-γ production, while L-1-MT had no effect. Stereospecific effect was also seen in PBMC tumoricidal activity, estimated by tumor cell viability (Trypan assay). The racemic mixture DL- and D-1-MT almost doubled the tumoricidal activity of PBMCs, while L-1-MT had no effect. These are previous unknown off-target effects of D-1-MT. Our data suggest the modulation of IFN-γ and the activation of tumor recognition and killing processes by immune cells as important features for the in vivo effects of the D-1-MT. These findings should be considered in future studies of immunoadjuvants for cancer treatment.