The induction of chromosomal aberrations in rat pleural mesothelial cells (RPMC) following in vitro treatment with chrysotile fibres has been demonstrated. The production of chromosomal aberrations was also observed after treatment of the cells with benzo-3,4-pyrene (BP). The yield of abnormal metaphases was dose-dependent and reached 58% at a BP dose of 2 micrograms/ml. Chrysotile fibres at 7 micrograms/ml induced 21% abnormal metaphases and the frequency decreased with further increases in fibre concentration. Their decline is possibly related to a lethal effect. Chrysotile-induced chromosomal aberrations were primarily of the chromatid type and included breaks and fragments. BP induced chromosome exchanges which were not seen following chrysotile treatment. Minutes and double minutes were detected in BP-treated RPMC and occasionally found after chrysotile application. These results confirm that chrysotile fibres are clastogenic for some cultured cells and demonstrate that the fibres induce chromosome damage in target RPMC.