A lead-intoxicated patient with extremely high blood lead levels and unexpectedly mild symptoms was studied prior to and following treatment with calcium disodium edetate (ethylenediaminetetraacetic acid) and then prior to and following oral administration of zinc sulfate. During chelation therapy, erythrocyte (delta)-aminolevulinic acid dehydratase (ALAD) activity decreased as blood lead levels fell. Urinary excretion of zinc increased and was more than 3.5 times greater than that of lead. The ratio of blood lead to serum zinc was greatest (1.47) when ALAD activity was lowest. Oral administration of zinc sulfate following chelation therapy resulted in a significant increase in mean ALAD activity. In vitro additions of zinc chloride to the patient's erythrocytes resulted in reactivation of ALAD activity. These studies suggest that zinc is an important element in the ALAD system in man. Zinc may play a protective role in lead toxicity, and zinc supplementation may be a useful adjunct to chelation therapy for lead toxicity.