Human mammary cells were examined for the presence of the high-molecular-weight surface glycoprotein fibronectin. Early passage mammary epithelial cell and fibroblast cultures from both carcinomas and normal tissues were tested for the presence of cell-associated fibronectin by immunofluorescence microscopy and for the synthesis and secretion of fibronectin by specific immunoprecipitation of metabolically labeled protein. In vivo frozen sections of primary carcinomas and normal tissues were tested for the localization of fibronectin by immunofluorescence microscopy. In contrast to the extensive fibrillar networks of fibronectin found in the fibroblast cultures, the epithelial cell cultures from both tissue sources displayed a pattern of cell-associated fibronectin characterized by powdery, punctate staining. However, the cultured epithelial cells, as well as the fibroblasts, secreted large quantities of fibronectin into the medium. Putative myoepithelial cells also displayed extensive fibrillar networks of fibronectin. The difference in cell-associated fibronectin distribution between the epithelial cells and the fibroblasts and putative myoepithelial cells provided a simple means of quantitating stromal and myoepithelial cell contamination of the mammary epithelial cells in culture. In vivo, normal tissues showed fibronectin primarily localized in the basement membrane surrounding the epithelial cells and in the stroma. Most primary carcinomas displayed powdery, punctate staining on the epithelial cells in addition to the fibronectin present in the surrounding stroma.