The present study was undertaken to investigate the involvement of oxidative damage in lead-induced toxicity in humans and to enlighten whether oxidative stress indicators are correlated with the known indices of lead toxicity. For these purposes, selected oxidative stress parameters along with some clinical indices of lead poisoning were determined in blood of battery plant workers and control subjects. Workers had significantly increased erythrocyte malondialdehyde (MDA) levels, catalase and glucose-6-phosphate dehydrogenase (G6PD) activities, and decreased blood glutathione:glutathione disulfide ratio compared to the controls. Increased blood lead concentrations and zinc protoporphyrin (ZPP) levels, and decreased delta-aminolevulinic acid dehydratase (ALAD) activity were used as clinical indices of lead toxicity. Statistically significant correlation between oxidative stress parameters and clinical indices implies that disrupted prooxidant/antioxidant balance might contribute to lead-induced toxicity in erythrocytes. A significant correlation was found between ALAD activity and blood lead levels in human subjects. Similarly significant correlation between ALAD activity and erythrocyte MDA concentrations was shown. Present data indicates that ALAD can serve as a valuable biomarker of oxidative stress in lead-exposed hematological system as well as being a biochemical indicator of lead exposure. (C) 2003 Elsevier Ireland Ltd. All rights reserved.