Large quantities of normal and malignant human mammary epithelial tissues are readily available as surgical discard material. We have developed culture conditions that permit long term, active proliferation of these HMEC in a serum-free medium. Thus, large pools HMEC can be stored frozen for repetition of experiments from the same individual's cell population, and for use of the same cell pool by multiple investigators. Of all the specimens that we have thus far examined, we have observed no instances of spontaneous transformation to immortality, nor any karyotypic abnormalities in the cells derived from reduction mammoplasties. However, exposure of normal HMEC to the chemical carcinogen, benzo(a)pyrene did lead to expression of an extended life in culture, and two instances of transformation to immortality. These two established cell lines contain some chromosomal abnormalities, yet retain a relatively stable karyotype upon continued passage in culture. Transformation to malignancy was achieved by exposing these cell lines to tumor viruses and oncogenes. Both the normal HMEC, and the HMEC transformed in vitro, are now being utilized to understand the factors controlling expression of mammary specific properties, response to and production of various growth factors, and the nature of the progressive events leading to malignancy. The maximal usefulness of this, and other human epithelial cell systems, for elucidating the mechanisms of normal and diseased human cellular physiology will require continued efforts to optimize the culture conditions so that they resemble as closely as possible the processes occurring in humans in vivo.