Human malignant pleural mesothelioma (MPM) is highly aggressive, and its prognosis is very poor. For an early diagnosis of MPM and developing new therapeutic strategies against the malignancy, it is necessary to better understand biological characteristics of MPM. In this study, we established two cell lines from pleural effusions derived from patients with MPM. Both cell lines expressed tumor markers of mesothelioma such as mesothelin, podoplanin, WT1, calretinin and keratin 5/6 whereas they did not express either CEA or TTF-1 which are often used as markers of lung adenocarcinoma. The cell lines harboured wild-type TP53, produced hyaluronic acid, and were not infected with SV40. When these two cell lines were cultured under hypoxia (1% O₂), they showed particular responses to the hypoxic condition, distinct from those to normoxic condition (21% O₂). Namely, the ability to form a colony originating from a single cell (plating efficiency and cloning efficiency) was stimulated under hypoxia in both cell lines. On the other hand, when the assays of cell growth were started at a relatively high cell density, the growth of both cell lines, regardless of anchorage-dependent or -independent, decreased under hypoxia. The differences of their growth between under hypoxia and under normoxia, and those depending on the cell density, may provide useful hints for developing a new strategy for diagnosis or therapy of MPM.