ASBESTOS is a serious occupational and environmental hazard because of its fibrogenic and carcinogenic actions on the lung. Asbestos fibres exist in various physical forms, all of which are toxic. It is not known, however, which form is the most hazardous or which fibre property is the most important for inducing the toxic effects. It is thus important to examine relationships between the structure of the fibres and their biological activity. We have investigated the relationship between the surface charge and the haemolytic activity (ability to rupture the erythrocyte membrane) of asbestos. We report here that different forms of asbestos can be activated or inactivated by surface charge alterations.