Glutathione transferase (GT) activity towards trans-stilbene oxide (tSBO), benzo[a]pyrene-4,5-oxide (B[a]PO) and 1-chloro-2,4-dinitrobenzene (CDNB) was measured in human liver and lymphocytes. GT-tSBO activity is catalyzed by GT mu which has polymorphic expression in human lymphocytes. Our results show that activity of GT-tSBO in lymphocytes correlates with its activity in liver (r = 0.7, P less than 0.001). GT activity towards BPO (GT-BPO) also correlated with GT-tSBO in lymphocytes and liver. However, interindividual variation of GT-BPO is less than that of GT-tSBO, suggesting that BPO may not be as specific a substrate for GT mu and therefore other GT isozymes may contribute to BPO conjugation. Conjugation of CDNB by GT was not different using cytosols from either high or low GT mu individuals. The functional significance of the GT-mu polymorphism was evaluated by measuring its effect on benzo[a]pyrene (B[a]P)- and aflatoxin B1 (AFB1)-DNA adduct formation in vitro. Human liver cytosols prepared from persons having low or high GT-tSBO activity were incubated with human liver microsomes, calf thymus DNA and B[a]P or AFB1. HPLC analysis revealed that the major B[a]P adduct was dG(N2)-7 beta, 8 alpha-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-dG). BPDE-dG adducts were decreased equally by cytosols from either low or high conjugators. In contrast, AFB1-DNA binding was inhibited to a greater extent in high conjugators than low conjugators. HPLC analysis demonstrates that adducts formed were AFB1-FAPyr and AFB1-N7-Gua. The correlation between AFB1-DNA adduct concentrations and GT mu activity was highly significant with a correlation coefficient of r = 0.88 at P less than 0.001. These results suggest that GT mu plays an important role in detoxifying DNA reactive metabolites of AFB1 and this enzyme may be a susceptibility marker for AFB1 related liver cancer. Moreover, our data demonstrate that lymphocytes are a reliable surrogate tissue for detecting liver GT mu polymorphisms.