1. The activity of erythrocyte δ-aminolaevulinic acid (ALA) dehydratase and blood protoporphyrin concentrations have been measured in patients with various anaemias, a group of subjects with known lead exposure and a group of control subjects. Leucocyte ALA synthase was measured in subjects from the last two groups.
2. Erythrocyte ALA dehydratase activity was significantly depressed in the group of lead-exposed subjects and showed a highly significant negative exponential relationship with blood lead concentration.
3. Blood protoporphyrin concentrations were significantly elevated in the group of lead-exposed subjects and patients with iron-deficiency anaemia and showed a significant positive exponential relationship with blood lead concentration.
4. Comparison of the least-squares regression analysis of these relationships and incidence of false positive and false negative results indicates that erythrocyte ALA dehydratase activity is a more accurate measure of environmental and moderate industrial lead exposure than blood protoporphyrin concentrations.
5. The correlations of erythrocyte ALA dehydratase and leucocyte ALA synthase activity, and of blood protoporphyrin concentrations and leucocyte ALA synthase activity, suggest that blood protoporphyrin more accurately reflects haem synthesis than does erythrocyte ALA dehydratase activity.