The extracellular matrix glycoprotein tenascin is limited to the periductal matrix of normal breast tissue but is markedly increased in both malignant and fibroadenomatous proliferations. It has been hypothesized that the changes in tenascin expression in these tissues are the result of epithelial induction of tenascin expression by the underlying mesenchyme. We have used Western and Northern blotting techniques to examine tenascin expression by normal and malignant mammary epithelial cells in culture. Normal mammary epithelial cells express tenascin in culture and incorporate the protein into the underlying matrix. The SV40-transformed mammary epithelial cell line HBL100 and some established mammary carcinoma cell lines also express tenascin. In contrast to normal mammary epithelial cells, carcinoma cells incorporate very little tenascin into the underlying matrix. To examine the source of tenascin expression in vivo, we have used in situ hybridization to demonstrate that the epithelial cells are a significant source of tenascin in both normal and malignant breast tissues.