Both interleukin-7 (IL-7) and IL-7 receptor (R) gene knockout (IL-7(-/-) and IL-7R(-/-)) mice were employed in order to directly investigate the importance of the IL-7 and IL-7R signaling pathway for the development of intestinal intraepithelial lymphocytes (IEL). Loss of the IL-7R-specific gene resulted in complete deficiency of the γδ T cell lineage with lack of Vγ4- and Vγ7-specific messages in the epithelium of the gastrointestinal (GI) tract in comparison to control mice of the same genetic background (~ 40%). Disruption of the IL-7-specific gene resulted in marked, but not complete depletion of γδ T cells (2-3%) in IEL. Furthermore, mRNA for both Vγ4 and Vγ7 genes were detected in the γδ IEL subset of IL-7(-/-) mice. The subtle differences between IL-7(-/-) and IL-7R(-/-)mice suggest that although IL-7 controls most of the expansion and/or development of γδ IEL, another ligand binding to the IL-7R also plays a discernable role. Furthermore, αβ IEL developed more slowly in IL-7R(-/-) mice when compared with ligand knockouts; however, the frequency of IEL T cells subsequently increased with age and normal levels of CD3+ T cells expressing the αβ TCR were detected by 2 and 3 months of age in IL-7(-/-) and IL-7R(-/-) mice, respectively. The direct comparison of IL-7(-/-) and IL-7R(-/-) mice clearly supports the hypothesis that both IL-7 and another IL-7R binding molecule can influence the development of γδ T cells in the intestinal epithelium.