The level of erythrocyte enzyme delta-aminolevulinic acid dehydratase (ALA-D) has been determined in conjunction with other standard tests for lead poisoning. The mean for 53 healthy persons was previously shown to be 175 ±38 units. Lead excretion rates above 1.0 mg/24 hr in response to 1 gm of edetate disodium calcium (Calcium Disodium Versenate) given intravenously were associated with depression of the enzyme activity to 60 units or less. Enzyme activity was maximally depressed when lead excretion exceeded 2 mg/24 hr. Activity of ALA-D was more sensitive and accurate in the diagnosis of lead poisoning than urinary ALA or coproporphyrin excretion and as good as blood or urine lead determinations. Activity of ALA-D is rapidly restored toward normal by treatment with edetate disodium calcium.