Necrotizing enterocolitis is a devastating inflammatory condition of the intestine that occurs almost exclusively in premature newborns. Although its exact pathogenesis is unclear, we have postulated that it may result from a predisposition of the immature intestine to mount an unusually robust and damaging response to microbial infection. In support of this idea, we report that the IL-8 response of an immature human enterocyte cell line to bacterial infection was significantly higher than that of a mature enterocyte cell line. The response in both cell lines was flagellin-dependent. Corresponding to the difference in IL-8 production, the immature enterocytes expressed appreciably lower levels of specific IκB genes when compared with the mature enterocytes. Similar developmentally regulated differences in cytokine response and IκB expression were also seen in primary rat enterocytes, indicating that these observations were not peculiarities of the cell lines. Furthermore, when the level of IκBα expression was increased in the immature cell line by transfection, the flagellin-dependent IL-8 response was attenuated. Thus, we have demonstrated a previously undescribed developmental regulation of IκB expression in the intestine involved in modulating the IL-8 response to bacterial infection, which may contribute to the pathogenesis of age-specific inflammatory bowel diseases such as necrotizing enterocolitis.