A sensitive, specific method for the measurement of uroporphyrinogen decarboxylase(porphyrinogen carboxy-lyase, EC4.1.1.37) in liver homogenates, haemolysates and other tissues is described. Pentacarboxylic porphyrinogen III, prepared from the corresponding porphyrin by reduction with sodium-mercury amalgam, is used as substrate. After oxidation and methyl esterification the reaction product, coproporphyrinogen III, is separated from the substrate and measured by high-performance liquid chromatography, with mesoporphyrin IX as internal standard. The method can readily be adapted for use with any of the other substrates of uroporphyrinogen decarboxylase, including uroporphyrinogens I and III.