Giardia ADI reduces PBMC proliferation through arginine consumption. The secreted Giardia protein ADI (GiADI) was expressed and purified from Giardia WB trophozoites. T-cells of human PBMCs of 5 blood donors were stimulated to proliferate by activating anti-CD3 and anti-CD28 (indicated by +) and the proliferation was assessed upon incorporation of 3H-labelled thymidine, as depicted in counts per minute (cpm) in logarithmic scale. T-cell stimulation was successful and not inhibited by buffer controls (+ GiADI buffer) or addition of heat denatured GiADI (+ GiADIb). GiADI (5 μg/mL) clearly reduced PBMC proliferation after T-cell specific stimulation, an effect that could be reversed by addition of arginine (+ GiADI + Arg) and partially also by its metabolite citrulline (+ GiADI + Citr). Significant differences are indicated by * (p < 0.5) and ** (p < 0.01).