Interleukin 6 (IL-6) is a pleiotropic inflammatory cytokine and its role in cancer is not yet clear.The effects of IL-6 on four breast cancer cell lines and normal mammary epithelium, cultured from milk were tested. Four different patterns of response to IL-6 were found depending on the differentiation status of the cells. In normal mammary epithelial cultures, the effects of IL-6 were mainly growth inhibitory, whereas in MCF-7, IL-6 had growth inhibitory and anti-adhesive effects. In T-47D and ZR-75-1 the anti-adhesive effects were prominent although the growth inhibitory effects were not. These anti-adhesive effects were associated with epithelioid to fibroblastoid morphological changes and a local decrease in E-cadherin expression. In the highly invasive cell line MDA-MB-231, which does not express E-cadherin, no effects of IL-6 were seen. IL-6 levels in the serum of 60 breast cancer patients were found to be increased in 27% (16/60) compared to 2% (1/50) in a control group. Furthermore, it was found that altered E-cadherin expression was seen in 69% of the primary tumours, although no significant association was found between raised serum IL-6 levels and altered E-cadherin expression. Finally IL-6 serum levels did not effect the survival of breast cancer patients. The authors therefore implicate IL-6 as a possible factor important in breast cancer progression and metastasis formation, although the clinical significance of this cytokine in breast cancer patients could not be established.