Background/Aims: Intestinal dysfunction observed during cow's milk allergy (CMA) is incompletely understood, and neither the effector cells nor the mediators responsible have been clearly identified. This study was undertaken to better characterize the implication of mononuclear cells in food allergy. Methods: Peripheral blood mononuclear cells (PBMC) from infants with CMA were cultured in the presence of cow's milk proteins (CMP), and the release of tumor necrosis factor α (TNF-α), Interferon gamma (IFN-γ), and interleukin 4 and 6 was measured. The effect of culture supernatants was tested on HT29 cl.19A intestinal cell monolayers mounted in Ussing chambers. Results: When stimulated by CMP, PBMC from infants with CMA released more TNF-α than those from control infants (429 ± 92 vs. 205 ± 34 pg/mL). Culture supernatants did not directly stimulate electrogenic chloride secretion by HT29 cl.19A cells, but epithelial barrier capacity was altered as shown by the significant decrease in electrical resistance (85 ± 17 vs. 135 ± 14 Ω·cm2 in controls) and the increases in intact horseradish peroxidase, [14C]mannitol, and 22Na+ fluxes. These effects were reversed when culture supernatants were neutralized with anti-TNF-α antibodies. Recombinant human-TNF-α altered the HT29 cl.19A epithelial barrier capacity, and its effect was highly potentiated by IFN-γ. Conclusions: These results indicate that during CMA, the high level of TNF-α released by mononuclear cells after milk protein challenge acts synergistically with IFN-γ to increase the intestinal permeability.