The association between treatment condition and micronucleus induction in Chinese-hamster lung fibroblasts, V79 cells, exposed to crystalline-silica (14808607) and glass fibers was examined under three different treatment conditions. V79 cells were cultured and incubated with silica doses of 17.2 to 275.8 micrograms per square centimeter (microg/cm2) and glass fiber doses of 1.7 to 27.6microg/cm2. Control cultures were exposed to solvent only. The cells were treated with a 24 hour exposure (24E), a 24 hour exposure plus 24 hour postincubation in fresh medium (24E/24P), or a 48 hour exposure (48E). Following treatments, the cells were prepared into slides using cytocentrifugation and examined microscopically for micronucleated cells (MNCs) and multinucleated cells (MTCs). The frequency of MNCs was significantly increased in V79 cells incubated with silica for 24 hours. The rise in MNC frequency was dose dependent only in cells of the 24E/24P group. Silica exposure caused a slight increase in the frequency of MTCs. A significant response in MTC frequency was only noted in the 24E/24P group exposed to a silica dose of 137.9microg/cm2. In contrast, glass fibers induced significant, dose dependent increases in both MNC and MTC frequencies under all three treatment conditions. The frequencies were most elevated in cells of the 24E/24P treatment group. The positive control N-methyl-N'-nitro-N-nitrosoguanidine elicited a significant response in all treatment groups. The authors conclude that 24E/24P is an appropriate treatment condition for dust and fiber exposures in the micronucleus assay.