Ac-erbB-2 expression vector was transfected into the estrogen receptor positive (ER+) MCF-7 human breast cancer cell line to determine if overexpression of this transmembrane tyrosine kinase could increase the malignant phenotype of this cell line. Loss of transfectedc-erbB-2 expression was observed when cells were carried in medium containing estrogen. Homogeneous populations stably overexpressing levels of the 185 kDac-erbB-2 observed in the SKBR-3 a breast cancer cell line which overexpressesc-erbB-2 as a result of gene amplification could be obtained by continually maintaining the transfected cell lines in estrogen-free conditions. Levels of constitutively activatedc-erbB-2 varied among clonal isolates. Whereas some over-expressing lines did acquire the ability to form transient tumor nodules in ovariectomized nude mice without estrogen supplementation, as well as in mice that received the antiestrogen tamoxifen, one cell line that exhibited the highest levels of constitutively activatedc-erbB-2 was able to form static tumors of a larger size under both conditions. This same cell line formed progressively growing tumors in estrogen-supplemented mice that were much larger than observed in mice injected with control cell lines, and also showed reduced sensitivity to antiestrogensin vitro, but it continued to have a low metastatic phenotype. These results suggest that signal transduction mediated by thec-erbB-2 tyrosine kinase can partially overcome the estrogen dependence of ER+ breast cancer cells for growth and thatc-erbB-2 overexpression confers a selective advantage to such cells in the absence of estrogen.