Intestinal glutathione S-transferase activities were measured during the GEEP Workshop in flounder Platichthys flesus collected from Langesundfjord, Norway. Though enzyme activities were generally higher in fish collected from 3 polluted sites by comparison with a reference site, the only significant differences in activities between the 3 sites ran counter to the anticipated pollution gradient. The results suggest that intestinal glutathione S-transferase activity may not be a sensitive indicator of pollution exposure under field conditions.