The endocellular incorporation of asbestos fibres seems to be capable to induce on Chinese hamster cell cultures numerical and structural chromosome aberrations. In the present study asbestos fibres have been added at different concentrations to human cultured lymphocytes which have been harvested at 48 and 72 h, respectively. The results do not show significant difference in the distribution of chromatid and chromosome-type aberrations between the control cultures and the cultures treated with asbestos.