A comprehensive listing of putative human breast carcinoma cell lines and the extent to which each has been characterized is presented. Criteria used to certify the human, mammary, and malignant origin of a cell line include: (a) a reliable histopathological diagnosis; (b) interspecies specificity established by human karyotype, isoenzyme profiles, and/or cell surface antigenicity; (c) intraspecies specificity, demonstrated by genetic evidence of a unique, human donor distinct from other cells including HeLa cells; and (d) organ specficity, supported by morphological evidence of epithelial structure and secretory activity, and especially by the expression of differentiated functions; these include presence of receptors for sex steroid hormones, hormone responsiveness, and production of milk proteins, fatty acids, or milk-specific antigens. Of the 47 cell lines for which data are here reported, 22 have been shown to be derived from human non-HeLa donors and to have epithelial morphology as revealed by light or electron microscopy. Differentiated function has been recorded for 19 cell lines. Additional human breast cancer cell lines have been reported, but characterization of some of these has been insufficient to judge the legitimacy of their predigrees. For others mammary origin is questionable. Six purported breast cell lines are in reality HeLa cells, and one is of nonhuman origin.