After affinity purification on immobilized glutathione, insect-cell-derived glutathione S-transferase (GST) fusion proteins contain variable amounts of protein contaminants of about 23-24 kDa. We have isolated these glutathione-binding proteins from the widely used Sf9 and Hi5 insect cell lines and characterized them by LC-MS and N-terminal sequencing. Based on the observation that these proteins have higher affinity for glutathione than GST fusions, we have found that by using differential elution conditions the amount of such contaminants in GST fusion preparations can be strongly reduced directly during the affinity purification step. The main interest of these results is that they are not restricted to a specific construct, but rather they seem to apply to various insect-cell-derived GST fusions.