The cytokinesis-block micronucleus (CBMN) assay has emerged as one of the preferred methods for assessing chromosome damage. Micronuclei (MN) are small, extranuclear bodies that are formed in mitosis from acentric chromosomal fragments or chromosomes that are not included in each daughter nucleus. Thus, MN contain either chromosomal fragments or whole chromosomes. The CBMN assay, together with a fluorescence in situ hybridization (FISH) technique using specific centromeric probes for chromosomes 7 and 8, were employed in mitogen-stimulated human lymphocytes pretreated with the benzene metabolite, 1,2,4-benzenetriol (BT). Treatment of human lymphocytes resulted in the induction of MN in a dose-dependent manner. The frequency of MN in control lymphocytes was 4.5 per 1000 binucleated (BN) cells and this increased to 9.5, 14, 28 and 40 per 1000 BN cells at 10, 25, 50 and 100 microM BT, respectively. The frequency of aneuploidy 7 and 8 in BN cells also increased at each concentration. Aneuploidy 8 was more frequent than aneuploidy 7, suggesting that chromosome 8 is more sensitive to aneuploidy induction by BT. The frequency of MN containing centromere positive signals for chromosomes 7 and 8 increased with the concentration of BT. The frequency of MN with centromere positive signals was higher for chromosome 8 than for chromosome 7, also suggesting a greater sensitivity of chromosome 8 to this agent. These results suggest that combined application of the CBMN assay with a FISH technique, using chromosome-specific centromeric probes, would allow the detection of aneuploidy in human lymphocytes and identify the mechanistic origin of MN induced by a clastogen or aneugen.