A model for the study of internalization of particles in mammalian cells was applied to asbestos and glass fibres. Briefly, a fluorescent fluid-phase endocytic marker, Lucifer Yellow CH (LY), was allowed to be incorporated into the lysosomal compartment of Syrian hamster embryo cells. Mineral fibres that were internalized by the cells subsequently became 'fluorescent', presumably when the fibre-containing endosome fused with the LY-containing lysosomes. This method was compared with differential interference contrast (DIC) optics. Approximately three times as many of the cell-associated fibres were determined to be internalized by the fluorescence method compared with DIC optics. Both fine and coarse glass fibres were internalized as effectively as asbestos fibres. The relative frequency of internalized (i.e. fluorescent) fibres increased until 4 hr after exposure compared with the total number of cell-associated fibres. The frequency of internalized fibres compared with the number of cell-associated fibres was constant over the range of fibre levels studied. A surface modification (octadecyldimethylchlorosilane-derivatization) of amosite fibres that decreased the carcinogenicity of the fibres, decreased slightly the number of internalized fibres relative to the number of cell-associated fibres, but this was not statistically significant. Cytoskeleton-interfering agents significantly decreased the relative number of internalized fibres.