Malignant mesothelioma is a tumor arising from serous surfaces and often related to asbestos exposure. Malignant mesothelioma is resistant to various forms of therapy. Radiotherapy, surgery or chemotherapy only slightly improve prognosis. IFN-γ produces complete or partial responses in stage-l patients with malignant mesothelioma. The in vitro biological effect of IFN-γ on malignant mesothelioma cells remains poorly elucidated. In the present study, 32 well-characterized human mesothelioma cell lines (HMCL) were treated with r-hu IFN--γ at 4 doses and cell growth was determined by a colorimetric method (MTT assay). Among the 32 HMCLs tested, 11 exhibited significant cell-growth inhibition; 16 were insensitive to r-hu IFN-γ, and 5 were slightly inhibited. The sensitive cell lines were strongly inhibited by r-hu IFN-γ. Our results show that HMCL exhibit a large range of responses to r-hu IFN-γ, some of which can be compared with those obtained in vivo in humans.