Loss of epithelial morphology and the acquisition of mesenchymal characteristics are typical for carcinoma cells in tumour progression. In human breast carcinomas, up-regulation of tenascin-C (TN-C) and vimentin (Vim) is frequently observed in cancer cells and correlates with increased malignancy. Thus, it is possible that TN-C is co-expressed with Vim, representing cancer cells that have undergone epithelial-mesenchymal transition (EMT). This study examined 128 breast carcinomas using immunohistochemical techniques to demonstrate that mammary cancer cells are a prominent source of both TN-C and Vim. Statistical analysis revealed a significant association between TN-C and Vim expression in cancer cells. TN-C expression also correlated positively with overexpression of c-erbB-2 oncoprotein and down-regulation of oestrogen receptors (ERs). Eleven human mammary cancer cell lines and two 'normal' cell lines were examined by western blotting and immunohistochemistry. Co-expression of TN-C and Vim was detected in the carcinosarcoma cell line HS 578T, SK-BR-3 (B), fibroblast-like MDA-MB-231 cells, and the myoepithelial cell line HBL 100. These findings suggest that TN-C and Vim, when co-expressed in mammary carcinoma cells, represent regulator genes likely to be involved in EMT during mammary carcinogenesis.