The membrane bound enzyme oxidizing protoporphyrinogen to protoporphyrin, a step in heme and chlorophyll synthesis, was purified to a single prominent polypeptide band on SDS/PAGE from barley mitochondrial fractions. It contained a variety of lipids including 0.66 mg of phosphatidyl ethanolamine and 0.46 mg of free fatty acid per mg of protein. Iron, but no flavins or cytochromes, was detected. In the presence of glutathione, enzymatic oxidation was inhibited by the iron chelator o-phenanthroline but was stimulated by iron EDTA. The purified enzyme was inhibited by reductants such as glutathione, ascorbate, NADH and NADPH. These findings are compatible with some direct or indirect involvement of lipids and iron in this oxidation in plants.