Fibres from weathered asbestos-cement products have little or no haemolytic activity, as compared with UICC chrysotile; this is probably the result of magnesium leaching during the weathering process. Weathered samples of asbestos cement are cytotoxic, but the release of lactic dehydrogenase (LDH) by guinea-pig alveolar macrophages caused by low and intermediate dust concentrations of UICC chrysotile is greater than that of such samples. The influence of serum is different as between UICC chrysotile and asbestos cement. In the former, LDH release by macrophages is enhanced, whereas it is reduced in the latter. Cytotoxicity is length-dependent in respect of LDH release from macrophages and proliferating cells, as well as cell proliferation. In all test systems, the sample from the unweathered core of an asbestos-cement plate is less toxic; only in the haemolysis system using an unbuffered solution can erythrocyte destruction be observed. This may be because this sample contains fewer single fibres than the others and because the specific surface of those fibres is smaller. The carcinogenicity of the weathered asbestos-cement chrysotile fibres is comparable to that of standard chrysotile fibres following intraperitoneal (i.p.) application.