The c-erbB-3 gene is a member of the EGF receptor family of membrane tyrosine kinases. Previous work has shown that the expression of this gene is frequently unregulated in human breast carcinoma-derived cell lines and in tumour biopsy material. As there is no evidence of gene amplification, we decided to examine the control of c-erbB-3 transcription in overexpressing cells. In this paper we describe the cloning of the c-erbB-3 promoter and its functional analysis within mammary-derived cell lines. We show that the promoter is more active in overexpressing cells and this is largely due to the action of the OB2-1 transcription factor which we have recently identified as mediating overexpression of the c-erbB-2 promoter in similar cell lines.