Receptors for the cytokines leukemia inhibitory factor (LIF), interleukin-6 (IL-6), oncostatin M (OSM), ciliary neurotrophic factor (CNTF) and interleukin-11 (IL-11) are members of the structurally conserved hemopoietin receptor superfamily. In addition, they all share the transmembrane signalling protein gp130. In this paper the expression and function of this family of receptors in breast cancer cells was examined. RT-PCR analyses demonstrated that gp130 was expressed in 12/12 breast cell lines and the specific receptor alpha-chains for IL-6, LIF, IL-11 and CNTF were expressed in the majority of these cell lines. This was in contrast to other hemopoietin receptors. Examination of 50 clinical samples of malignant breast tissue by RT-PCR showed a similar pattern of expression of gp130 associated receptors. Treatment of breast cancer cell lines with OSM resulted in changes in cellular morphology. Cellular proliferation was inhibited following exposure to OSM (3/4 cell lines), IL-11 (2/4 cell lines), and by IL-6 and LIF (1/4 cell lines). Cell surface binding of LIF and OSM was also documented. The expression of these receptors in 12/12 cell lines and greater than 95% of clinical samples suggests that these molecules may be important in regulating the growth of breast cells.