An immunohistological study in the human breast and the rodent breast (from inbred Ludwig/Wistar/Olac rats) was conducted with the use of a murine monoclonal antibody, which reacts with the common acute lymphoblastic antigen, a glycosylated polypeptide of a molecular weight of 100,000. The epitope, as recognized by this antibody, is expressed on myoepithelial cells of the normal human and rat breasts and was studied in the developing rodent mammary gland. Ultrastructural studies in the normal human breast clearly demonstrated the presence of the antigen on the lateral membrane of the myoepithelial cells with no staining of luminal cells, blood vessels, or stromal elements. The antigen survived prolonged enzymatic digestion of human breast tissue and could be demonstrated on myoepithelial cells in single-cell suspensions of human breast where it stained approximately 3-14% of the total cell population. The presence of this antigen on myoepithelial cells is discussed in the context of myoepithelial differentiation in the breast and the potential utility of the antibodies for cell separation.