Abstract.δ‐Aminolaevulinic acid dehydratase activity is traditionally accepted as the most sensitive measurable biological index of lead toxicity. We have measured δ‐aminolaevulinic acid dehydratase activity and blood lead concentration in 47 healthy controls (A), 42 iron deficient patients (B) and 38 occupationally exposed to lead subjects (C). Blood lead levels [x(SD)] did not differ between groups A and B [0.51 (0.21) and 0.43 (019) μmol L‐1, respectively] while those of group C [2.28 (0.56) μmol L‐1were significantly higher (P<0.001) as compared to the controls. δ‐Aminolae‐vulinic acid dehydratase activity [x(SD)] was significantly increased [3599 (1909) μmol L‐1h‐1] in group B and decreased in group C [1052 (532) μmol L‐1h‐1] as compared to the controls [2034 (446) μmol L‐1h‐1] (P<0.001). There was a significantly negative correlation of logarithm of δ‐aminolaevulinic acid dehydratase with lead in both groups B (P<0.05) and C (P<0.001) but not in group A (P= 0.1). δ‐Amino‐laevulinic acid dehydratase activity had a high specificity (100%) but a low sensitivity (37%) as an index of toxic lead exposure. According to our data the value of δ‐aminolaevulinic acid dehydratase measurement in the diagnosis of lead intoxication is doubtful in cases with low blood lead levels, while in the presence of iron deficiency its reliability is further reduced, since low blood lead levels may be falsely predicted. δ‐Amino‐laevulinic acid dehydratase activity should be restricted only to monitori