The present studies were aimed at determining if the use of a cell culture medium that supports proliferation of human mammary epithelial cells of the luminal lineage would allow routine isolation of breast cancer cells from primary and metastatic tumor specimens. Results obtained with mammary epithelial cells derived from reduction mammoplasty specimens and primary breast carcinomas indicated that growth of cells on type I collagen-coated dishes in Ham's F-12 medium supplemented with insulin, hydrocortisone, epidermal growth factor, cholera toxin, and 5% fetal bovine serum resulted in the growth and serial passage of cells that stained positively for the luminal cell marker cytokeratin 19. By contrast, growth of mammary epithelial cells in a growth factor-supplemented serum-free medium resulted in the emergence of mammary epithelial cell colonies that were uniformly negative for keratin 19. Filter isolation methods were used to isolate individual keratin-19-positive colonies from primary cultures derived from breast cancer specimens. All of the luminal mammary epithelial cells isolated from breast cancer tissues expressed characteristics of normal cells. Keratin-19-positive colonies isolated from several different tumors all grew rapidly for 30 to 60 days in culture and then senesced. Cells were isolated from one tumor that was known to have undergone a loss of heterozygosity at a specific locus in the p53 gene. All colonies isolated from this specimen contained both p53 alleles, which was consistent with their origin from normal luminal cells. Cells were also isolated from one tumor in which the c-erbB2 protein was drastically overexpressed in the neoplastic cells. Once again, keratin-19-positive colonies isolated from this tumor did not overexpress the c-erbB-2 protein. Experiments were then performed with cells derived from pleural effusions and metastatic lymph nodes. Results obtained with these specimens indicated that the growth conditions that support the growth of normal luminal mammary epithelial cells do not support the growth of neoplastic cells. However, the omission of cholera toxin, epidermal growth factor, and type I collagen substratum resulted in the isolation of two long-term cell lines. Both cell lines have population doubling times of approximately 100 h, are hyperdiploid, and stain positively for cytokeratin 19. Thus, culture conditions that support the growth of normal luminal mammary epithelial cells do not, in general, support the growth of breast cancer cells.