The oxidation of protoporphyrinogen IX to protoporphyrin IX in yeast cells is enzyme-dependent. The enzyme, protoporphyrinogen oxidase, associated with purified mitochondria isolated from Saccharomyces cerevisiae was solubilized by sonic treatment in the presence of detergent and partially purified. The molecular weight of the enzyme was 180,000 plus or minus 18,000. The purified preparation could be stored at -20 degrees in the presence of 20% glycerol for several months without loss of activity. Enzyme activity was destroyed by heating above 40 degrees and by proteolytic digestion and irreversible inactivation occurred outside the pH range of 4.0 to 9.5. The pH optimum of the enzymic reaction was 7.45 and the value of the Michaelis constant was approximately 4.8 muM. Protoporphyrinogen oxidase did not catalyse the oxidation of coproporphyrinogen I or III or uroporphyrinogen I or III and the rate of enzymic oxidation of mesoporphyrinogen IX was less than 20% of that observed with protoporphyrinogen IX. The presence of thiol groups in the enzyme system was indicated but no metal ion or other cofactor requirement was demonstrated. Enzyme activity was insensitive to cyanide, 2,4-dinitrophenol, and azide whereas it was inhibited in the presence of Cu-2+ or Co-2+ ions, high ionic strength, heme, or hemin.