It has been known for some years that lead and some other metals such as mercury and silver have an inhibitory action on the activity of δ-aminolaevulinic acid dehydratase (ALAD) in red blood cells. Zinc in small concentrations has likewise an inhibitory effect on ALAD, while, in higher concentrations, this metal gives a significant increase in the activity of ALAD in red blood cells. This was shown in in vitro as well as in vivo studies.