Spontaneous and diepoxybutane (DEB)-induced sister-chromatid exchanges (SCEs) were examined in whole-blood lymphocyte cultures of 3 men and 4 women. A strong increase in mean number of SCEs per cell with increasing DEB concentrations (0, 2, and 4 microM) was observed in cultures of all subjects, but 3 of the donors were clearly more sensitive than the others. The SCE measurements were repeated 2-6 times per donor over a period of 55 months to assess the stability of the individual SCE response. The results showed that SCE induction by DEB was steady in the individuals during the follow-up at each DEB dose, with no significant differences among the repeated experiments. At 4 microM DEB, the DEB-sensitive and -resistant donors could be reliably be differentiated from each other in all trials. As DEB-sensitivity has been suggested to be due to the lack of glutathione S-transferase (GST) T1, the donors were genotyped for the presence of GSTT1 and GSTM1 genes. The 3 individuals found to be DEB-sensitive were all of the GSTT1 null genotype, whereas the 4 DEB-resistant donors were GSTT1 positive, which supported the role of the GSTT1 gene in determining DEB-sensitivity. Three of the DEB-resistant and none of the DEB-sensitive had the GSTM1 null genotype. Thus, the lack of the GSTM1 gene was not associated with the DEB-sensitivity trait. In conclusion, the present findings show that individual SCE responses to treatment of cultured human lymphocytes with DEB can reliably be reproduced in repeated trials. The results confirm that the GSTT1 gene but not the GSTM1 gene is important in determining individual sensitivity to the in vitro genotoxicity of DEB.