Tcf-4 is a member of the Tcf/Lef family of transcription factors that interact functionally with β-catenin to mediate Wnt signaling in vertebrates. We have previously demonstrated that the tumor suppressor function of APC in the small intestine is mediated via regulation of Tcf- 4/β-catenin transcriptional activity. To gain further insight into the role of Tcf-4 in development and carcinogenesis we have generated several mouse monoclonal antibodies, one of which is specific for Tcf-4 and another of which recognizes both Tcf-3 and Tcf-4. Immunohistochemistry performed with the Tcf 4-specific monoclonal antibody revealed high levels of expression in normal intestinal and mammary epithelium and carcinomas derived therefrom. Additional sites of Tcf-3 expression, as revealed by staining with the Tcf- 3/-4 antibody, occurred only within the stomach epithelium, hair follicles, and keratinocytes of the skin. A temporal Tcf-4 expression gradient was observed along the crypt-villus axis of human small intestinal epithelium: strong Tcf-4 expression was present within the crypts of early (week 16) human fetal small intestine, with the villi showing barely detectable Tcf-4 protein levels. Tcf-4 expression levels increased dramatically on the vim of more highly developed (week 22) fetal small intestine. We conclude that Tcf- 4 exhibits a highly restricted expression pattern related to the developmental stage of the intestinal epithelium. The high levels of Tcf-4 expression in mammary epithelium and mammary carcinomas may also indicate a role in the development of this tissue and breast carcinoma.