Hyaluronan is one of the main components of the extracellular matrix. It is synthesized at the cell plasma membrane by specific hyaluronan synthases (HAS). Although a large number of studies have described hyaluronan in pleural effusion from malignant mesothelioma, the source of hyaluronan in malignant mesothelioma has been subject to controversy.The mRNA expression of all three HAS in malignant mesothelioma cells was studied using RT-PCR. The hyaluronan production in culture medium of malignant mesothelioma cells was also examined using high-performance liquid chromatography (HPLC).We found that 9/10 malignant mesothelioma cell lines and one primary culture of malignant mesothelioma cells expressed HAS-1, while 10/10 malignant mesothelioma cell lines and one primary culture of malignant mesothelioma cells expressed HAS-2 and HAS-3. In addition, we demonstrated hyaluronan in the culture medium of 6 out of 10 malignant mesothelioma cell lines and one primary culture of malignant mesothelioma cells.Our results show that malignant mesothelioma cells express all three HAS and synthesize hyaluronan. The expression of HAS isoforms and hyaluronan in malignant mesothelioma cells in cultures and previous observations by other investigators indicate that these cells are, at least in part, responsible for hyaluronan synthesis in vivo.