The breast cancer-associated protein pS2 is also present in many human gastrointestinal tumors. In contrast to breast carcinomas, gastrointestinal tumors do not express estrogen receptors, indicating that the expression of pS2 is not estrogen-dependent. The pS2 expression was analyzed in 14 adenosquamous tumors of the human gastrointestinal tract. The aim was to investigate if the cell type specific localization of pS2 was limited to the glandular part. The data clearly confirm such a specific compartmentation of the pS2 expression, suggesting pS2 to be a secreted protein. Due to the specific expression, pS2 may become a new and useful diagnostic marker of adenocarcinoma.