Tumor microenvironment is an important factor, which sustains and promotes the tumors by inflammatory signals. Interleukin-6 (IL-6) is known as a multifunctional cytokine, which is a major activator of the signaling pathway of Janus kinases (JAKs)/signal transducer and activator of transcription 3 (STAT3). In this study, we aimed to investigate the effect of IL-6 in the tumor microenvironment on carcinogenesis. For this purpose, healthy breast tissue-derived stromal cells (HBT-SCs) and malign breast tissue-derived stromal cells (MBT-SCs) were co-cultured with MCF-7 (human breast adenocarcinoma cell line) cells using semipermeable membranes. The cell proliferation was monitored with water-soluble tetrazolium (WST) and carboxyfluorescein succinimidyl ester (CFSE) assays. Protein levels were measured by enzyme-linked immunosorbent assay (ELISA) and Western blot hybridization, while gene expressions were measured by real-time PCR. The results demonstrated that IL-6 protein levels increased significantly in the supernatants of MBT-SCs when they were co-cultured with MCF-7 cells. In accordance with this, the expression of IL-6 was significantly higher in MBT-SCs. Additionally, the expression of STAT3 in MCF-7 cells increased slightly when they were co-cultured with MBT-SCs. Considering together, there is an important interaction between tumor microenvironment and tumor cells mediated by IL-6 signaling. Thereby, the targeting on IL-6 signaling in the treatment of cancer might effectively prevent the tumor progression.