This study aimed to determine whether drug treatment produced structural changes in porphyrinogen carboxy-lyase enzymatic protein, leading to altered properties. Rat-liver enzyme was obtained from normal animals and from those with hexachlorobenzene (HCB)-induced porphyria, and several of its properties were comparatively studied. The enzymes from both sources were purified 110-fold. They were similar in subcellular distribution, ammonium sulfate fractionation, calcium phosphate gel adsorption, storage stability, requirements of incubation media, effects of salts and photo-oxidizing agents. The enzymes differed with respect to effect of incubation temperature, pH, aerobiosis, chelating agents, dithiothreitol, methylene blue, heat stability and chromatographic behaviour on DEAE-cellulose and Sephadex G-100. These differences would appear to indicate structural differences between the two types of enzymatic protein. Since the porphyrinogenic action of HCB can be mediated through a metabolite, structural differences could arise through the binding of an inhibitory metabolite to the whole enzyme or through modifications to the protein during its synthesis.