Four experiments in which SPF Wistar rats were inoculated intrapleurally with asbestos or other materials are described. Mesotheliomata were observed in a considerable proportion of animals with all the samples of asbestos used and with a sample of brucite. A few were produced with synthetic aluminium silicate fibres and single ones with barium sulphate, glass powder and aluminium oxide. The risk of developing a mesothelioma at a given time after injection was approximately proportional to the dose. Of the UICC standard reference samples, crocidolite was the most carcinogenic and removal of the oils by benzene extraction did not alter the carcinogenicity of these samples. Chemical properties also seem unlikely to be the main factor producing mesotheliomata but the results support the hypothesis that the finer fibres are the more carcinogenic, and this is additional to the known aerodynamic advantage which the finer fibres have in penetrating to the periphery of the lung.