Cytosolic glutathione S-transferases were purified from the epithelial cells of human small and large intestine. These preparations were characterized with regard to specific activities, subunit and isoenzyme composition. Isoenzyme composition and specific activity showed little variation from proximal to distal small intestine. Specific activities of hepatic and intestinal enzymes from the same patient were comparable. Hepatic enzymes were mainly composed of 25 kDa subunits. Transferases from small intestine contained 24 and 25 kDa subunits, in variable amounts. Colon enzymes were composed of 24 kDa subunits. In most preparations, however, minor amounts of 27 and 27.5 kDa subunits were detectable. Separation into isoforms by isoelectric focusing revealed striking differences: glutathione S-transferases from liver were mainly basic or neutral, enzymes from small intestine were basic, neutral and acidic, whereas large intestine contained acidic isoforms only. The intestinal acidic transferase most probably was identical with glutathione S-transferase Pi, isolated from human placenta. In the hepatic preparation, this isoform was hardly detectable. The specific activity of glutathione S-transferase showed a sharp fall from small to large intestine. In proximal and distal colon, activity seemed to be about equal. In the ascending colon there might be a relationship between specific activity of glutathione S-transferases and age of the patient, activity decreasing with increasing age.