We have recently reported the molecular characterization of a novel murine receptor PTK (myk-1), belonging to the eph-related family, whose expression was differentially regulated during mammary gland development and elevated in invasive mouse mammary tumours. In this communication we have investigated the cellular origin of myk-1 expression by in situ hybridisation and RNase protection. Murine antisense myk-1 probe specifically recognised ductal and alveolar epithelium in the resting mouse mammary gland. In normal human breast, the expression of htk, the human homologue, was also confined to the secretory luminal epithelial cells. RNase protection analysis of enriched luminal and myoepithelial cells prepared from human reduction mammoplasty tissue confirmed the luminal specificity. Elevated expression of htk was found in several human breast carcinoma cell lines as well as in primary, high grade, infiltrating, ductal carcinomas of the breast. The specific epithelial expression of the myk-1/htk receptor PTK suggest a specialised role of this eph-related PTK in the differentiation and/or maintenance of secretory epithelium in the adult.