Intraperitoneal (i.p.) injection of crocidolite asbestos was used to induce mesotheliomas in rats. The morphological changes of the mesothelial cells were studied by light and electron microscopy and by cytologic examination of peritoneal washings. After injection, the asbestos fibres stimulated an acute inflammatory response and were rapidly phagocytosed by the mesothelial cells and incorporated into the submesothelial tissues. At 7 days, the normal microvillous surface of the mesothelium was replaced with a syncytium of proliferating mesothelial cells showing extensive loss of microvilli. Nine months or so later, multifocal mesothelial tumours arose within the peritoneal cavity. The surface thermodynamic properties of normal, asbestos-stimulated mesothelial cells and of mesothelial tumour cells in culture were studied using an aqueous two-phase system containing 4% Dextran T-2000 and 4% poly (ethylene glycol) (PEG) w/w. After asbestos stimulation, there was a significant (P < 0.01) increase in contact angle between the dextran-rich phase and the mesothelial cell surface. These changes were even greater for the mesothelial tumours. The results indicate that the work of adhesion for asbestos-stimulated mesothelial cells and mesothelial tumours is lower than in normal tissue. These findings may be relevant to the process of tumour spread in the serosal cavities and to the development of distant metastases.