A cell line designated "HMMME" was established from the pleural fluids of a malignant mesothelioma patient. This line grew well without interruption for 12 years and was subcultured over 200 times. The cells were spindle and roundish in shape and displayed a monolayer sheet in an epithelial pavement cell arrangement. They were neoplastic, had pleomorphic features, and easily formed multilayering without contact inhibition. The cell cytoplasm was strongly positive against anti-vimentin, anti-calretinin and anti-pan-keratin, but negative against anti-BerEP4. The cells proliferated rapidly, and the population doubling time was about 42 hours. Their chromosome number showed a wide distribution of aneuploidy with a mode in the diploid range; many marker chromosomes were observed. The cultured cells were easily transplanted into the subcutaneous of nude mice and produced a tumor classified as a malignant mesothelioma.