1. The activities of six of the enzymes of haem biosynthesis have been assayed in peripheral blood from patients with lead poisoning, acute intermittent porphyria or hereditary coproporphyria.

2. Compared with normal subjects the lead-poisoned subjects had highly significant depression of δ-aminolaevulinate dehydratase, coproporphyrinogen oxidase and ferrochelatase.

3. Lead-poisoned subjects had highly significant elevation of δ-aminolaevulinate synthase activity.

4. δ-Aminolaevulinate synthase activity was inversely related to the haemoglobin concentration.

5. Increased δ-aminolaevulinate synthase and decreased δ-aminolaevulinate dehydratase activity are also found in acute intermittent porphyria.

6. Increased δ-aminolaevulinate synthase, normal porphobilinogen deaminase and uroporphyrinogen decarboxylase and decreased coproporphyrinogen oxidase are found in both lead poisoning and hereditary Coproporphyria.

7. These enzyme changes explain the recognized patterns of porphyrins and porphyrin precursors in blood and urine in these conditions.

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