Fosfomycin exerts anti-inflammatory effects through inhibiting the production of proinflammatory cytokines. Transcription of the genes for these proinflammatory cytokines is regulated by NF-kappaB. We tested the hypothesis that fosfomycin inhibits the activation of NF-kappaB induced by tumor necrosis factor-alpha (TNF-alpha) in human monocytic U-937 cells, and a T cell line (Jurkat). Western blot analysis demonstrated that fosfomycin inhibits NF-kappaB activation in both cells. Flow cytometry revealed that fosfomycin suppresses NF-kappaB activation in both cells in a dose-related manner. These findings are consistent with the idea that fosfomycin suppresses the production of proinflammatory cytokines via inhibition of NF-kappaB activation.