The initial phase of growth of the parenchymal component of the mouse mammary gland is ductal clongation, which is mainly accomplished by proliferating cells in a specialized structure termed end bud. End buds are composed of multiple layers of epithelial cells (so called body cells) which are capped by a single layer of morphologically unique cells termed cap cells. We sought to examine the interrelationship between cap cells and other epithelial cell subclasses using a variety of antibodies to different keratin proteins and also antibodies to vimentin, actin and collagen IV. An extensive immunohistochemical characterization of the epithelial components of the developing and differentiating mammary gland demonstrated that cap cells were devoid of any immunohistochemically-detectable keratins but were positive for collagen IV. In contrast, the majority of cells in the end bud along with the luminal epithelial and myoepithelial cells were keratin positive. The body cells of the end bud were the only cells which were positive for antibody to keratin 6, a keratin which previously has been reported to be expressed in proliferating mammary epithelial cells. In addition, estrogen receptor was localized only to epithelial cells of ducts, alvcoli and body cells of end buds, but not to cap cells or myoepithelial cells. We interpret these results to suggest that cap cells are not totpotent stem cells but rather cells specialized in paving the way for ductal elongation as well as serving as precursors to myoepithelial cells.