Members of the c-erbB family have been implicated in poor prognosis in breast cancer. Given the propensity for heterodimerisation within the erbB family, the pattern of co-expression of these receptors is likely to be as functionally important as aberrant expression of any given receptor alone. Therefore, the patterns of expression of the receptors, epidermal growth factor receptor (EGF-R), c-erbB-2, c-erbB-3, c-erbB-4, and one of the erbB ligands, heregulin (HRG), were examined in normal and malignant breast cell lines and compared with expression of oestrogen receptor (ER), a classical indicator of good prognosis. There was an inverse correlation between ER and EGF-R mRNA levels, as previously described, but no correlation between either of these receptors and c-erbB-2. c-erbB-3 expression was positively correlated with ER. In contrast, HRG expression was inversely related to ER. Expression of antisense-ER resulted in increased EGF-R mRNA, demonstrating a functional link between the expression of these 2 genes, however, there was no significant change in c-erbB-2 or c-erbB-3 mRNA, suggesting that ER is not directly involved in control of expression of these genes. A comparison of individual erbB receptors and HRG revealed that the majority of lines expressing increased levels of c-erbB-2 also expressed elevated levels of c-erbB-3 mRNA, and none of the cell lines that expressed both c-erbB-2 and either c-erbB-3 or c-erbB-4 expressed the ligand HRG. In summary, the levels of expression of c-erbB-1, -2, -3, and -4 varied in this series of breast cell lines, and the pattern of expression and the relationship of each growth factor receptor to the expression of ER was quite distinct. The lack of expression of HRG in cell lines that express receptors may be indicative of paracrine interactions between erbB ligands and their cognate receptors and may suggest that the ligand and receptors are expressed in different subtypes of breast epithelial cells from which the cell lines are derived.