Members of the Ets family of winged helix-loop-helix transcription factors play important roles in the development and function of multiple mammalian cell lineages. Elf-1 is an Ets-related transcription factor that is expressed at high levels in T cells and is known to regulate the expression of several T cell genes, including the granulocyte-macrophage colony stimulating factor (GM-CSF) gene, the interleukin-2 receptor alpha subunit (IL-2Ralpha) gene, and the CD4 gene. In the studies described in this report, we have characterized the pattern of expression of Elf-1 in the developing mouse embryo and in adult mouse tissues as well as in multiple immortalized human and murine cell lines. Elf-1 is expressed at high levels throughout thymocyte development, with equivalent levels of Elf-1 expression seen in all subsets of maturing thymocytes and T cells. Somewhat surprisingly, however, Elf-1 is also expressed at high levels in epithelial cells lining the oral cavity, the lung, the CNS, and the gastrointestinal and urinary tracts as well as in the skin of the developing mouse embryo and at lower levels in the adult mouse testis and liver. Western blot analyses of a large number of immortalized cell lines demonstrated high-level Elf-1 expression in T and B lymphocyte and macrophage cell lines as well as in two prostate carcinoma cell lines. Low-level expression was observed in fibroblasts, embryonic stem cells, and myoblasts. Taken together, our data suggest that in addition to its role in regulating T cell development and function, Elf-1 may regulate gene expression in the B cell and myelomonocytic lineages, as well as in multiple epithelial cell types during murine embryonic development.