Normal mammary tissue, mammary tumors, and 16-day embryos (of BALB/c strain mice) were dissociated into single cells. These were plated at high (5×105 cells/cmt surface area), medium (1×105 cells/cm2), and low (1×104 cells/cm2) densities in a defined culture medium supplemented with fetal calf serum, insulin, and hydrocortisone. Cell shape, total DNA, DNA synthesis, and incidence of mitoses were determined and compared in the various cultures during the ensuing weeks. Epithelial cells seeded at high density grew to confluence and produced 3-dimensional multicellular structures called domes. Dome cells were similar to the surrounding monolayer cells in terms of DNA production and mitosis. Epithelial cells seeded at intermediate density grew to confluence but never produced domes. Epithelial cells seeded at low density became very spread out and thin, with many polyploid nuclei, and never became confluent. In the transplanted tumors used, more than 60% of the nuclei were initially 4C; these rapidly reached higher ploidies in low density culture. In contrast, nuclei of normal cells were uniformly 2C, and became polyploid only after several days in culture at low density. Cultures of normal and tumor-derived epithelial cells were similar in other respects at all 3 densities. In contrast to these cultured epithelia, embryo fibroblasts cultured under the same conditions grew to high densities of predominantly diploid cells at all the plating densities used. These observations are discussed in terms ot properties or fibroblasts versus epithelia and of normal versus neoplastic epithelia in culture.