Hyaluronan (HA) is a nonsulfated glycosaminoglycan that is secreted in significant quantities by pleural mesothelial cells (PMC) and malignant mesothelioma cells (MMC). The functional significance of HA deposition in the pleural space has not been fully elucidated. In this study, we hypothesized that low molecular weight but not high molecular weight hyaluronan induces proliferation and migration of MMC, and that the hyaluronan receptor (CD44S) expressed on the mesothelioma cell surface is involved in this process. We evaluated the effect of low molecular weight hyaluronan (LMWHA) and high molecular weight hyaluronan (HMWHA) on four MMC lines (CRL-2081, CRL-5915, CRL-5830, CRL-5820) proliferation and haptotactic migration. We also studied the expression of HA receptor CD44S on MMC by Northern hybridization and flow cytometry. The binding of LMWHA and HMWHA to MMC surface was determined by FACS analysis using FITC-conjugated hyaluronan. Our results indicate that the MMC line that expressed the highest amount of CD44 receptor showed increased proliferation and haptotactic migration of MMC when stimulated with LMWHA but not HMWHA. Monoclonal antibody against CD44 inhibited proliferation by about 12-40% and migration by 10-35% in the MMC lines that were studied, and thus in part inhibited LMWHA-induced proliferation and migration in MMC. LMWHA binding to MM cell surface was significantly higher than HMWHA. This directly correlated with their CD44 receptor expression. Neutralization of CD44 receptor significantly reduced the LMMHA binding to MMC. These results provide evidence that the interaction between the adhesive protein receptor CD44 and extracellular matrix component (HA) transmits regulatory signals for mediating the locomotion and proliferation of MMC, and thus plays an important tole role in localized extension of tumor growth.