Large amounts of Na⁺ can be accumulated in the skin upon HSD (high salt diet, 4% NaCl, 0.9% saline) without commensurate water retention, resulting in an increase in interstitial fluid tonicity which is paralleled by infiltration of macrophages into the Na⁺-overloaded interstitium. Whether interstitial hypertonicity modulates immune function and thereby the anti-infective capacity of the host, is unknown. We hypothesized that local hypertonicity augments immune response and thereby interstitial clearance of pathogens. To test this hypothesis, we investigated activation and anti-infective capacity of bone marrow-derived macrophages (BMMs) exposed to osmotic stress. We cultured BMMs under standard medium conditions (LS), and with increased NaCl concentration (+40 mM; HS) in the presence or absence of the Toll-like receptor 4 (TLR4) ligand lipopolysaccharide (LPS). We observed that osmotic stress in concert with LPS, but not osmotic stress alone, augmented macrophage activation, including increased iNOS-activity. Leishmania major (L. major) causes a chronic skin infection, and its healing is critically dependent on iNOS activity. Therefore, we analyzed whether augmented BMM-activation under osmotic stress conditions resulted in enhanced leishmanicidal capacity. We found that activation of L. major-infected BMMs in the presence of HS not only synergistically augmented iNOS mRNA expression, but also NO production. Furthermore, activation of infected BMMs under HS conditions significantly reduced the infection rate and intracellular parasite burden, whereas HS alone without additional TLR4 stimulation had no effect on the cell's leishmanicidal capacity. We conclude that Na⁺-mediated hypertonicity of the internal microenvironment augments TLR4-mediated activation of BMMs and thereby increases their anti-infective capacity. Our data suggest that internal environment composition plays a hithertofore underestimated role in the host's ability to combat infections.