Glutathione-S-transferases (GSTs) have a function in xenobiotic metabolism. They are a significant multifunctional family with a wide variety of catalytic activities. In the current study, we determined in vitro inhibition effects of 2,4-dichlorophenoxyacetic acid dimethylamine salt (2,4-D DMA), haloxyfop-P-methyl, glyphosate isopropylamine, dichlorvos, and -cyhalothrin on purified GST. For this purpose, GST were purified from Van Lake fish (Chalcalburnus tarichii Pallas) liver with 29.25 EUmg(-1) specific activity and 10.76% yield using GSH-agarose affinity chromatographic method. The pesticides were tested at various concentrations on in vitro GST activity. K-i constants were calculated as 0.17 +/- 0.01, 0.25 +/- 0.05, 3.72 +/- 0.32, 0.42 +/- 0.06, and 0.025 +/- 0.004mM, for 2,4-D DMA, haloxyfop-P-methyl, glyphosate isopropylamine, dichlorvos, and -cyhalothrin, respectively. -Cyhalothrin showed a better inhibitory effect compared to the other pesticides. The inhibition mechanisms of -cyhalothrin were competitive, while the other pesticides were noncompetitive.