The study objectives were to assess the ability of intratracheal injection methods to discriminate between nine fibre types in respect of pulmonary biopersistence, and to provide approximate estimates of relative biopersistence and durability for a study of general relationships with biological and toxicological responses. The test fibres included six samples of size-selected fibre types specially prepared for research purposes, two commercially available fibres, and amosite. A 1 mg dose of each fibre type was administered to rats by intratracheal injection. The relative biopersistence of fibres in different size categories was assessed from the changes in mean lung burden, as determined by electron microscopy, at 3 days and 1, 6 and 12 months after injection. The ability of the test materials to resist dissolution was measured in a parallel series of simple in vitro acellular experiments at two pHs and in a continuous flow dissolution test. The observed differences in the persistence of fibres of differing length recovered from rat lungs were consistent with the current hypothesis that short fibres are cleared by cellular processes and long fibres by dissolution and disintegration. Differences in persistence of long (> 20 microns) fibres were correlated with measured rates of dissolution in vitro. Differences in persistence among those fibre types also studied by others workers were consistent with their findings after inhalation and intratracheal injection. Overall, the differences in the biopersistences of the test fibres following intratracheal injection were sufficient to enable an examination of the relationship of biopersistence with other biological and toxicological responses. Biopersistence was influenced by both fibre dimensions and solubility.