A set of inhibitors for rat liver microsomal glutathione transferase have been characterized. These inhibitors (rose bengal, tributyltin acetate, S-hexylglutathione, indomethacin, cibacron blue and bromosulphophtalein) all have I50 values in the 1-100 microM range. Their effects on the unactivated enzyme were compared to those on the N-ethylmaleimide- and trypsin-activated microsomal glutathione transferase. It was found that the I50 values were decreased upon activation of the enzyme (5-20-fold), except for S-hexylglutathione, where a slight increase was noted. Thus, the activated microsomal glutathione transferase is generally more sensitive to the effect of inhibitors than the unactivated enzyme. It was also noted that inhibitor potency can vary dramatically depending on the substrate used. The I50 values for the N-ethylmaleimide- and trypsin-activated enzyme preparations are altered in a similar fashion compared to the unactivated enzyme. This finding indicates that these two alternative mechanisms of activation induce a similar type of change in the microsomal glutathione transferase.