Background: Recently, there has been a great deal of interest in the role of bacterial products in altering the allergic immune response in early life. Objective: We sought to develop a model of bacterial inflammation in human subjects to characterize the ex vivo response to LPS in the nasal mucosa of young nonatopic children. Methods: Using the explant technique, we cultured nasal mucosa in the presence or absence of increasing concentrations of LPS (0.001-1 μg LPS/mL) for 24 hours. Immunocytochemistry and in situ hybridization were used to phenotype the cells and cytokines in the explant, and immunofluorescence was used to study proliferation of T cells in explants cultured with LPS and 5-bromo-2′-deoxyuridine (BrdU). Results: CD68, CD3, tryptase, elastase, IFN- γ IL-12, and IL-10 immunoreactivity was detected in unstimulated tissue. LPS stimulation caused a dose-dependent increase in the number of CD3⁺ (n = 7, P <.05), CD68⁺ (n = 7, P < .01), elastase-positive (n = 7, P < .01), tryptase-positive (n = 7, P < .05), IL-2 –positive (n = 6, P < .05), IL-12 –positive (n = 6, P < .05), and IFN- γ–positive (n = 6, P < .05) cells compared with media alone. LPS had no effect on major basic protein, IL-4, IL-5, or IL-13 immunoreactivity. LPS (0.1 μg/mL) caused an increase in the number of BrdU-positive cells as early as 2 hours after incubation (n = 8, P < .001). The maximal BrdU immunoreactivity was seen 24 hours after incubation with LPS, at which time approximately 70% of BrdU-positive cells were CD3⁺ T cells. Conclusions: Our results suggest that LPS is capable of inducing local inflammation, T_H1 cytokine immunoreactivity, and proliferation of CD3⁺ activated T cells in the absence of de novo cell recruitment. This model holds promise for future studies investigating the importance of bacterial products in regulating local immune function in young atopic children. (J Allergy Clin Immunol 2002;110:771-6.)