In this study, we show that conditions previously found to promote the selective growth of human breast epithelial cells (HBEC) in serum-free primary cultures established from normal or malignant tissue can be extended to cultures initiated at low seeding densities (< 5000 cells/cm2). The epithelial nature of the cells produced was documented by their positive staining with antibodies specific for keratins 8, 14, and 18, and 2 antibodies that recognize epithelial-specific antigens (Ber-EP4 and HB8630). HBEC growth was not affected, either positively or negatively, by the use of a medium containing a combination of fetal calf and horse serum, which promotes the growth of many types of stromal cells and associated hematopoietic precursors, or by the inclusion in the initial cell suspension of marrow cells at HBEC to marrow cell ratios typical of bone marrow samples from patients with metastatic breast cancer. The presence of fibroblast feeders from a variety of sources enhanced the growth of HBEC to different degrees. In cultures initiated with low numbers of cells obtained from samples of breast carcinoma, HBEC growth was generally reduced by comparison to cultures of normal HBEC. With the detection methods used, it was not possible to determine the extent to which this decreased growth was due to a reduced frequency of malignant HBEC with in vitro precursor activity, or the presence of reduced numbers of residual normal HBEC precursors, or both. However, preliminary data indicate that this approach also allows the detection of some breast carcinoma cells with proliferative ability that are present in the marrow or pleural effusions of some breast cancer patients. These studies demonstrate the feasibility of detecting normal and malignant HBEC with growth potential when these are cultured at low density and/or as rare contaminants of marrow cell suspensions, and provide a starting point for their further characterization.