Tenascin is an extracellular matrix glycoprotein that is not present in the normal mature rat mammary gland. The distribution of tenascin was examined by immunohistochemistry in mammary tumors from carcinogen-treated and untreated rats, in virus-induced mammary tumors from mice, and in a variety of mammary gland lesions from humans. Tenascin was detectable in the stroma of the malignant but not of the benign tumors from all species. An inhibition ELISA, testing homogenates of rat tumors, confirmed that tenascin was present in malignant but not in benign tumors. Thus, tenascin was consistently found to be a stromal marker for epithelial malignancy in the mammary gland. It is concluded that tenascin may be involved in the interactions between the epithelial and mesenchyme-derived (stromal) components of the mammary gland, which are known to influence epithelial carcinogenesis in this organ.