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Gallium is commonly used in the semiconductor industry and medical field. Biologically, gallium is able to interrupt iron metabolism. Exposure to gallium has been shown to affect the human immune system. The purpose of this study was to investigate the in vitro biological effects of different gallium concentrations on cultured human peripheral blood mononuclear cells (PBMCs) in terms of cell growth, cytokine release, and apoptosis induction. In addition, the in vivo effects of gallium were analyzed by Wistar rat model. Our results revealed that low concentrations (1–10 μg/ml) of gallium promoted cells to enter the S phase of cell cycle and enhanced cellular release of tumor necrosis factor-α, interleukin-1β, and interferon-γ, both in vitro and in vivo. In contrast, high concentrations of gallium (50–100 μg/ml) induced apoptosis. Furthermore, gallium-induced cytokine release and apoptosis could be inhibited by iron-saturated transferrin (Tf–Fe). These results suggest that the concentration-dependent effects of gallium on PBMCs are related to iron metabolism.