Abstract. An entire mammary epithelial outgrowth, capable of full secretory differentiation, may be comprised of the progeny of a single cellular antecedent. This conclusion is based upon the maintenance of retroviral insertion sites within the somatic DNA of successive transplant generations derived from a single mammary fragment. In addition, dissociation of these clonal dominant glands and implantation of dispersed cells at limiting dilution demonstrated that both duct-limited and lobule-limited outgrowths were developed, as well as complete, fully differentiated glands. Thus, transplantation has revealed three distinct mammary epithelial progenitors in the mouse. Similar studies have extended this observation to rat mammary tissue. Recently, using cre-lox conditional activation of reporter genes, a new epithelial progenitor, specific for mammary secretory epithelium in postlactation females has been uncovered. In situ, these cells were shown to regenerate secretory lobules upon successive pregnancies. In transplant studies, they demonstrated the capacity for self-renewal and contributed to the new generation of all of the known epithelial cell types among mammary epithelium. In limiting dilution, the parity-induced progenitors were capable of engendering lobule-limited and duct-limited outgrowths in their entirety, but not completely developed glands. Serial transplant studies indicate that these progenitors have a significant but limited capacity for self-renewal.