The role of the glutathione S-transferase T1 gene (GSTT1) in determining genotoxic response to l,2:3,4-diepoxybutane (DEB), an epoxide metabolite of 1,3-butadiene, was studied by analysis of micronuclei (MN) in cultured human lymphocytes using the cytokinesis block method. Fluorescence in situ hybridization (FISH) with an alphoid satellite DNA probe specific for the centromeres of all human chromosomes was applied to identify MN harboring whole chromosomes. Whole-blood lymphocyte cultures of 11 GSTM1 (glutathione S-transferase M1)-positive individuals (i.e. having at least one GSTM1 allele), of whom six were GSTT1-positive (with at least one GSTT1 allele) and five GSTT1-null (GSTT1 homozygously deleted), were treated for 48 h (starting 24 h after culture initiation) with two different concentrations (2 and 5 mM) of DEB. The GSTT1-null individuals were excessively sensitive to DEB, showing, on average, ∼2.5 times higher induced MN frequency (control frequency subtracted) than the GSTT1-positive donors, both at 2 mM (mean/1000 binucleate cells 29.8 versus 11.8, P < 0.05) and 5 mM (87.6 versus 34.0, P < 0.001) DEB. In accordance with the known strong clastogenicity of DEB, MN without centromeric FISH signals were particularly increased, the difference between the two GSTT1 genotypes being statistically significant at both concentrations of DEB (mean induced MN/1000 binucleate cells 23.1 versus 9.9, P < 0.05, at 2 mM; 69.7 versus 24.2, P < 0.001, at 5 mM). In addition, centromerepositive (C+) MN were induced, suggesting that DEB also has some aneuploidogenic activity. The GSTT1-null genotype showed a significantly (P < 0.05) higher mean frequency of induced C+ MN than the GSTT1-positive genotype, at both 2 (6.7 versus 1.9) and 5 mM (17.9 versus 9.8) DEB. At the higher dose mean nuclear division index was lower in the GSTT1-null group (1.80) than in the GSTT1-positive group (2.05, P < 0.01). These findings support earlier results from the analysis of sister chromatid exchange showing that individual sensitivity to the genotoxic and cytotoxic effects of DEB is largely explained by lack of the GSTT1 gene.