Objectives of this study were to determine if impurities in technical grade benfuracarb inhibit glutathione-S-transferase and amidase and to identify structures of impurities in technical grade benfuracarb. Technical grade benfuracarb, active ingredient, and impurity inhibited glutathione-S-transferase, and their $I_{50}$ were $9.7{ imes}10^{-4}M,;>1.0{ imes}10^{-3}M,;1.8{ imes}10^{-4}M$, respectively. Such inhibition, however, was not higher than that by ethacrynic acid, a selective inhibitor to GST. Technical grade benfuracarb, active ingredient, and impurity also inhibited amidase, and their $I_{50}$ were $6.0{ imes}10^{-5}M,;4.3{ imes}10^{-4}M,;7.6{ imes}10^{-5}M$, respectively. Our results show that the inhibition of both detoxifying enzymes by impurities in benfuracarb was 10-fold lower than that by active ingredient, suggesting that both active ingredient and impurities are involved in the inhibition of both detoxifying enzymes. Of four impurities (IM $1{sim}4$) that were separated from technical grade benfuracarb, IM 2 and IM 3 inhibited GST and amidase. Based on data from IR, $^1H$-NMR, $^{13}C$-NMR and MS, it was determined that IM 2 is ethyl-N-isopropylamino propionate and IM 3 is ethyl-N-isopropyl-N(chlorosulfenyl)aminopropionate.