Abstract The δ-aminolevulinic acid (ALA) dehydratase activity in erythrocytes, the urinary output of ALA, coproporphyrin and lead, and the level of lead in the blood were determined simultaneously in workers exposed to lead poisoning. The decrease of ALA dehydratase activity in lead poisoning (0.263 ± 0.081 μmoles PBG/ml erythrocyte/h as compared with 0.920 ± 0.162 for normal value) correlates very closely ( p < 0.01) with the raised blood level of lead and urinary output of ALA, and significantly ( p < 0.05) with the duration of exposure to lead. However, poor correlations were found between the decrease in ALA dehydratase activity and the increased urinary outputs of coproporphyrin and lead. Erythrocytes obtained from patients with other neurological and hematological disorders showed normal activity of ALA dehydratase. Reduced glutathione was effective for the recovery of the decreased ALA dehydratase activity in lead-poisoned erythrocytes in vivo. It is concluded that the determination of ALA dehydratase activity in erythrocytes offers an excellent measure for the evaluation of lead poisoning and that administration of reduced glutathione seems to be useful for treating patients with lead-poisoning.