Recent association studies suggest that genetically determined deficiencies in GSTs might be a risk factor for idiosyncratic adverse drug reactions resulting from the formation of reactive drug metabolites. hGSTP1-1 is polymorphic in the human population with a number of single nucleotide polymorphisms that yield an amino acid change in the encoded protein. Three allelic variants of hGSTP1-1 containing an Ile105Val or Ala114Val substitution, or a combination of both, have been the most widely studied and showed different activity when compared to wild-type hGSTP1-1*A (Ile105/Ala114). In the present study, we studied the ability of these allelic variants to catalyze the GSH conjugation of reactive metabolites of acetaminophen, clozapine, and diclofenac formed by bioactivation in in vitro incubations by human liver microsomes and drug metabolizing P450 BM3 mutants. The results show that effects of the change of amino acid at residue 105 and 114 on conjugation reactions were substrate dependent. A single substitution at residue 105 affects the ability to catalyze GSH conjugation, while when both residue 105 and 114 were substituted the effect was additionally enhanced. Single mutation at position 114 did not show a significant effect. The different hGSTP1-1 mutants showed slightly altered regioselectivities in formation of individual GSH conjugates of clozapine which suggests that the binding orientation of the reactive nitrenium ion of clozapine is affected by the mutations. For diclofenac, a significant decrease in activity in GSH-conjugation of diclofenac 1',4'-quinone imine was observed for variants hGSTP1-1*B (Val105/Ala114) and hGSTP1-1*C (Val105/Val114). However, since the differences in total GSH conjugation activity catalyzed by these allelic variants were not higher than 30%, differences in inactivation of reactive intermediates by hGSTP1-1 are not likely to be a major factor in determining interindividual difference in susceptibility to adverse drug reactions induced by the drugs studied. © 2013.