Since the advent of transmission electron microscopy of tissues capable of growth and regeneration, cell and developmental biologists have postulated that the undifferentiated cells observed within these tissues represent tissue-specific stem or progenitor cells. However, no studies have addressed the issue of whether these undifferentiated, putative stem cells persist in growth senescent tissues. Serially transplanted mammary epithelium consistently displays growth senescence beginning at the third transplant generation. This process is not uniform throughout the transplanted population and complete growth quiescence for all portions of a given outgrowth is reached subsequent to the 6th transplant generation. Mammary epithelial cells bearing the morphological characteristics of undifferentiated stem cells likewise disappear from senescent populations simultaneous with growth cessation. In premalignant mammary epithelial populations, which exhibit indefinitely prolonged growth potential, both of these cell types are maintained. This observation provides further support for the conclusion that these ultrastructurally distinct mammary cells represent the mammary stem/progenitor cell population.