Deposition of fibronectin (FN) was studied by indirect immunofluorescent staining, employing monospecific rabbit anti-human FN antibody, in the following 3 systems: cultures of cells derived from Rous sarcomas of chickens and of normal chick fibroblasts; frozen sections of chicken Rous sarcomas; and frozen sections of mouse and human mammary tissue, normal and neoplastic. The normal fibroblasts produced a well-oriented, delicate, fibrillar FN network. The sarcoma cells in culture also formed a FN-positive matrix, but one very different in appearance, consisting of coarse, unoriented strands. FN was also present abundantly between the cells of the sarcomas in vivo. In normal mammary tissue, FN was localized predominantly in the basal lamina region, well delineated from the surrounding FN-reactive stroma. The epithelial cells were generally, but not always, FN-negative. In apparently normal tissue areas of the breasts of mammary carcinoma patients, epithelial cells with highly reactive cell peripheries were frequently seen; the stroma surrounding the positive cells was often poor in FN and the basal lamina region lacked the substance. Mammary carcinoma cells were almost always negative, but it was characteristic of these tumours that the surrounding stroma displayed FN richly.