Abstract: The glutathione S-transferase (GST) were affinity purified in both mice treated with 200 mg kg-1 benzo (a) pyrene once weekly for 8 weeks and control mice. The affinity purified enzymes were isoelectric focused and three zones were selected in each group of mice. GST in the BaP treated mice were focused into zone I (pI 7.5 ), zone II (pI 8.4) and zone III (pI 9.4). In control mice, the zones were zone I (pI 8.4), zone II (pI 9.2 ) and zone III (pI 9.8. The affinity purified and focused enzymes were subjected to SDS-PAGE and assayed for substrate specificities. The SDS-PAGE of the affinity purified GST revealed 3 subunits of molecular radius of approx. 23, 24.5 and 26 kD in control mice and 24, 26.5 and 28 kD in BaP treated mice. The activities of the focused GST towards 1-chloro-2,4-dinitrobenzene (CDNB) were similar in both BaP treated and control mice. Activities of focused enzymes in the BaP treated mice towards 3,4-dichloronitrobenzene (DCNB) and ethacrynic acid (EA) were higher than control mice. The findings suggested that the GST in BaP treated mice were different to that in control mice. The SDS-PAGE showed that the subunits in the BaP treated mice were larger than control mice.