An assay for human erythrocyte glutathione-S-transferase is described. The procedure is both sensitive and reproducible; sampling and storage conditions are also investigated. Reference values are given for normal neonates, children and adults. The normal red cell glutathione-S-transferase activity decreases significantly during the first weeks of life and remains constant afterwards. A slight but significant increase is observed over 75 years. Sex difference has no influence on enzyme activity. A significant increase (up to fourfold) in red cell glutathione-S-transferase activity is noted in newborns with hyperbilirubinemia exceeding 135 mumol/l. Red cell glutathione-S-transferase is also significantly increased in hemodialysed subjects suffering from renal dysfunction.