AIM: To investigate the mechanism by which IL-6 is involved in cancer prognosis, and further to demonstrate the relationship between IL-6 and tumor-associated antigens such as CA15-3, CEA and CA125 in breast cancer. METHODS: In the present study, we transfected an exogenous IL-6 gene into the MCF-7 cells. Secretion of CA15-3, CEA and CA125 into the culture media were measured by Enzyme Linked Immunosorbent Assay (ELISA). RESULTS: After a 72 hours in culture, the amount of IL-6 in the media of pCI-neo-IL-6-transfected MCF-7 cells (338.5±22.6 pg/106 cells) was significantly higher than that of non-transfected MCF-7 cells (25.4±4.6 pg/106 cells, P＜0.01, paired t-test), or pCI-neo-transfected MCF-7 cells (19.6±3.0 pg/106 cells, P＜0.01, paired t-test). The levels of CA15-3, CEA and CA125 secreted by the pCI-neo-IL-6-transfected MCF-7 cells were significantly higher than that of the parental MCF-7 cells or pCI-neo-transfected MCF-7 cells. The specific IL-6 antibody could decrease the expression of CA15-3, CEA and CA125 in both the MCF-7 cells and the IL-6 cDNA-transfected MCF-7 cells. CONCLUSION: Transfer of IL-6 gene augments tumor-associated antigens of human breast cancer cells. The association of elevated IL-6 concentration with a poor prognosis in cancer patients may partially be the result of increased expression of tumor-associated antigens by IL-6.