Background: We have previously shown that the anti-tumor activity of human lymphocytes is diminished in vitro after 12-hours pre-exposure to simulated microgravity (SMG). Here we used an immunocompromised mouse model to determine if this loss of function would extend in vivo, and to also test the efficacy of IL-2 and zoledronic acid (ZOL) therapy as a potential countermeasure against SMG-induced immune dysfunction. We adoptively transferred human lymphocytes that were exposed to either SMG or 1G-control into NSG-Tg (Hu-IL15) mice 1-week after they were injected with a luciferase-tagged human chronic myeloid leukemia (K562) cell line. Tumor growth was monitored 2x weekly with bioluminescence imaging (BLI) for up to 6-weeks. Results: Mice that received lymphocytes exposed to SMG showed greater tumor burden compared to those receiving lymphocytes exposed to 1G (week 6 BLI: 1.8e{}^{10} \pm 8.07e{}^9 versus 2.22e{}^8 \pm 1.39e{}^8 photons/second; p 0.0001). Peak BLI was also higher in the SMG group compared to 1G-control (2.34e{}^{10} \pm 1.23e{}^{10} versus 3.75e{}^8 \pm 1.56e{}^8 photons/second; p = 0.0062). Exposure to SMG did not affect the ability of human lymphocytes to engraft or evoke xeno-graft-versus-host disease in the mice. Additionally, we injected the mice with IL-2 and zoledronic acid (ZOL) to expand and activate the anti-tumor activity of NK cells and \gamma \delta-T cells, respectively. This treatment was found to revive the loss of anti-leukemic function observed in vivo when lymphocytes were pre-exposed to SMG. Conclusions: Microgravity plays a contributory role in loss of tumor control in vivo. Immuno-stimulating agents like ZOL+IL-2 may offer an important countermeasure for immune dysregulation during prolonged spaceflight.