CXCL12 is a chemokine that binds to a G-protein-coupled receptor (CXCR4). CXCL12 is expressed in various tumors and is considered as playing an important role in tumor growth and invasion. The aim of this study is to investigate the expression of CXCL12 in human malignant mesothelioma (MM), the chemotactic effect of CXCL12 derived from MM, and the expression of CXCR4 in MM tissues in relation to regulatory T cells. CXCL12 expression was examined by immunostaining of tissue specimens from malignant pleural mesothelioma (MPM) and malignant peritoneal mesothelioma (MPEM). The MM group comprised 6 patients (4 men/2 women, MPM=4, MPEM=2, aged 56.0 +/- 12.4 years) and the control (non-mesothelioma) group also had 6 patients (4 men/2 women aged 65.0 +/- 6.7 years). CXCL12 mRNA expression was also examined by RT-PCR in MPM cell lines (H28, H2052, and H2058), while CXCR4 mRNA expression was examined by in situ hybridization in MPM tissue. CXCL12 was expressed in the cytoplasm of MM cells from all patients, but was not expressed in the control group. H2052 and H2058 cells expressed CXCL12 mRNA, but H28 cells did not. CXCL12 in MM tissue homogenate supernatant had a chemotactic effect on CXCR4-expressing THP-1 cells. CXCR4 mRNA was expressed by a part of LCA+CD3+ Foxp3+CD25+ T cells that were located adjacent to the border of CXCL12-expressing epithelioid MPM. These findings suggest that CXCL12 contributed to tumor-related inflammation by inducing the accumulation of CXCR4-expressing cells with regulatory T cell markers around MM.