Progesterone has been used in the treatment of patients with recurrent or metastatic progesterone receptor-positive endometrial carcinoma and breast cancer. In vitro study using a breast cancer cell line, T47D, demonstrated an increase in p53 gene expression and induction of apoptosis by the administration of progesterone. Therefore, we investigated the effect of progesterone administration on the proliferation and apoptosis in a mesothelioma cell line, 211H. The expression of the progesterone receptor gene was detected in this cell line by a nested RT-PCR method. The proliferation of the cell line was suppressed after a 10-day incubation with 30 microM progesterone. In progesterone-treated 211H cells, apoptotic cells were detected by TUNEL assay and nuclear DNA fragmentation analysis. These results clearly demonstrated that progesterone administration suppressed the cell proliferation and induced apoptosis in malignant mesothelioma cells.