A mouse mammary epithelial cell line with morphogenetic properties in vivo, Comma-Dβ, was used to isolate and to characterize mammary progenitor cells. We found that a homogeneous cell population expressing high surface levels of stem cell antigen 1 (Sca-1) was able to give rise in vivo to ductal and alveolar structures comprising luminal secretory and basal myoepithelial cells. Unlike the Sca-1^high, the Sca-1^neg/low cell population displayed a reduced morphogenetic potential. The Sca-1^high cells presented moderate CD24, high CD44 and α6 integrin surface levels, expressed basal cell markers p63, keratins 5 and 14, but no luminal and myoepithelial lineage markers. In culture, the Sca-1^high cells generated identical daughter cells that retained their in vivo developmental potential, indicating that these cells were maintained by self-renewal. Plated at clonogenic density in Matrigel, Sca-1^high cells formed spheroids that included luminal and myoepithelial cells. Thus, the isolated Sca-1^high basal cells possess several features of stem/progenitor cells, including specific markers, self-renewal capacity, and the ability to generate the two major mammary lineages, luminal and myoepithelial. These data provide evidence for the existence of basal-type mouse mammary progenitors able to participate in the morphogenetic processes characteristic of mammary gland development.