Understanding the diverse pathological processes which are initiated in the ultimate organizational units of the mammary parenchyma--the lobules and ductules--will to a large extent be based on understanding the normal breast. During the routine diagnostic evaluation of 11 surgical specimens for a variety of benign and malignant breast lesions, grossly normal mammary parenchyma, subsequently confirmed as histologically normal by light microscopy, was sampled for this purpose. Tissue was studied by histology, light microscope immunohistochemistry, transmission electron microscopy, scanning electron microscopy, and ultrastructural immunohistochemistry. A number of features hitherto disregarded in the literature are described in detail, or are elaborated with respect to earlier descriptions. These include: the presence of solitary cilia and lipid-rich residual bodies in intralobular fibroblasts, and the frequent association of inflammatory cells (lymphocytes, plasma cells, macrophages, mast cells) with the cell bodies and processes of fibroblasts. For the first time also, CD34, best known as an endothelial cell marker, has been demonstrated by ultrastructural immunohistochemistry on the cell surface of fibroblasts. Finally, scanning electron microscopy has demonstrated new features of the intercellular matrix in which the fibroblasts and inflammatory cells are located. The significance of these findings is discussed in terms of a postulated immune surveillance role of the mononuclear cells in collaboration with the fibroblasts.