Mesothelial cells, the progenitor cells of the asbestos-induced tumor mesothelioma, are particularly sensitive to the toxic effects of asbestos, although the molecular mechanisms by which asbestos induces injury in mesothelial cells are not known. We asked whether asbestos induced apoptosis in mesothelial cells and whether reactive oxygen species were important. Rabbit pleural mesothelial cells were exposed to crocidolite asbestos or control particles (1-10 micrograms/cm2) over 24 hr and evaluated for oligonucleosomal DNA fragmentation, loss of membrane phospholipid asymmetry, and nuclear condensation. Asbestos fibers, not control particles, induced apoptosis in mesothelial cells by all assays. Induction of apoptosis was dose dependent; crocidolite (5 micrograms/cm2) induced apoptosis (15.0 +/- 1.1%, mean +/- SE; n = 12) versus control particles (< 4%), as measured by appearance of nuclear condensation. Apoptosis induced by asbestos, but not by actinomycin D, was inhibited by extracellular catalase, superoxide dismutase in the presence of catalase, hypoxia (8% oxygen), deferoxamine, and 3-aminobenzamide (an inhibitor of the nuclear enzyme, poly(adenosine diphosphate-ribosyl) polymerase). We conclude that asbestos induces apoptosis in mesothelial cells via reactive oxygen species. We speculate that escape from this pathway could allow the abnormal survival of mesothelial cells with asbestos-induced mutations.