The chemical and physical properties of serpentine and amphibole asbestos are considered in the context of their interaction with tissue of the tracheobronchial tree and lungs. In vitro studies in cultures of several types are evaluated and work with the erythrocyte hemolysis system is reviewed. Although fibers of the two major mineral types differ substantially, it is likely they are modified by secretions and membranes of cells after inhalation to the respiratory tract. Investigations using virgin asbestos might not provide an accurate picture of events in vitro.