This report describes the isolation and characterization of a new human breast cancer cell line, SUM-102PT, obtained from a minimally invasive human breast carcinoma. SUM-102PT cells have a near diploid karyotype, and early-passage cells had minor chromosomal abnormalities including a 5, 12 and a 6, 16 reciprocal translocation. The cells were isolated and have been continually cultured in three defined media, one of which contains exogenous epidermal growth factor (EGF). SUM-102PT cells have also been carried in an EGF-free medium supplemented with progesterone. All SUM-102PT cells require EGF receptor (EGFR) activation for continuous growth, because incubation of the cells with EGFR-neutralizing antibodies or with EGFR kinase inhibitors blocks growth of these cells. Southern analysis indicates that the EGFR gene is not amplified in these cells; however, these cells express high levels of EGFR mRNA. Thus, SUM-102PT is representative of a class of human breast cancers characterized by high level EGFR expression in the absence of gene amplification. SUM-102PT cells cultured in EGF-free, progesterone-containing medium express high levels of constitutively active EGFR. Conditioned medium from SUM-102PT cells contains an EGF-like mitogen that binds to a heparin-agarose affinity matrix with high affinity. Northern analysis for various EGF family members indicates that SUM-102PT cells synthesize heparin binding (HB)-EGF mRNA. HB-EGF protein is detectable on the surface of these cells by immunohistochemistry, and SUM-102PT cells are killed by diphtheria toxin, which acts by binding to HB-EGF. Furthermore, HB-EGF antibodies partially neutralize the mitogenic activity of the conditioned medium. Thus, EGFR activation in SUM-102PT cells is mediated, at least in part, by autocrine/juxtacrine stimulation by HB-EGF. SUM-102PT cells also express constitutively active STAT-3 homodimers. Constitutively tyrosine-phosphorylated STAT-3 homodimers were also detected in another breast cancer cell line, MDA468, which has an EGFR amplification and also has constitutive EGFR activity. Thus, SUM-102PT is a new human breast cancer cell line that expresses activated EGFR as a result of an autocrine/juxtacrine interaction with HB-EGF which, in turn, results in activation of STAT-3.