delta-Aminolaevulinate dehydratase activity was measured in liver biopsy specimens from 13 patients with porphyria cutanea tarda, 13 alcoholics and 10 control subjects. The mean enzyme activities in liver of porphyria cutanea tarda and alcoholics were 34% (P less than 0.001) and 49% (P less than 0.001) respectively of the control level. Heat treatment of liver supernatant caused slight inhibition of the enzyme activity in porphyria cutanea tarda and in controls. Addition of Zn2+ to liver supernatants slightly increased the enzyme activity in both porphyria cutanea tarda and controls. The addition of liver supernatant from the porphyria cutanea tarda to purified bovine delta-aminolaevulinate dehydratase did not result in suppression of the enzyme activity. The apparent Km value of delta-aminolaevulinate dehydratase was 4.3 X 10(-4) mol/l for porphyria cutanea tarda livers and 3.3 X 10(-4) mol/l for control livers. The difference between the two was not significant.