The drugs of the class avermectins areantiparasitic agents, which are widely used in medical and agricultural fields, especially in veterinary medicine. The aim of this study was to investigate the inhibitory effects of avermectin derivatives such as abamectin, doramectin, eprinomectin, ivermectin, and moxidectin, which are used for internal and external mammalian parasites. Glutathione S-transferase (GST, E.C. 188.8.131.52) was purified from fresh human erythrocytes. The purification of the GST enzyme was performed separately by affinity chromatography with a yield of 34.81% and 117.94-fold purification. The control of the pure GST enzyme was performed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and a single band was obtained. The IC50 values were approximately 0.31, 0.39, 0.13, 0.44, and 0.73mM for abamectin, doramectin, eprinomectin, ivermectin, and moxidectin, and the K-i values were 0.32 +/- 0.06, 0.39 +/- 0.09, 0.13 +/- 0.03, 0.44 +/- 0.02, 0.73 +/- 0.04mM, respectively. This data revealed that the tested avermectins showed significant inhibitory effects on the GST enzyme.