Fibronectin and types I, III and IV collagens were localized by an indirect immunofluorescence technique in 10 fibro-adenomas and 27 mammary adenocarcinomas. In the benign lesions, a densification of a type III collagen meshwork was noted around galactophoric channels. In carcinomas, various patterns of connective tissue proteins distributions were observed depending upon the histological types of lesions. Connective tissue proteins are restricted to the perivascular areas in mucinous carcinomas. Type III collagen is largely distributed in the stroma of poorly differentiated tumors. In differentiated carcinomas, discontinuous linear deposits of type IV collagen or fibronectin are localized at the border of epithelial proliferations. They probably represent the remmants of epithelial basement membrane. In intra- canalar adenocarcinomas, type IV collagen and fibronectin form irregular and discontinuous depositifs . These finding indicate that extensive connective tissue remodelling is observed during neoplastic infiltration. The type of stroma and connective tissue proteins distributed in these epithelial proliferations depends primarily upon the histological type of malignant infiltrating carcinomas.