Antioxidative inhibition by protoporphyrin (PP) of peroxidative damage in lysosomes, mitochondria and microsomes of rat liver was investigated at 24 h after an intravenous administration of PP. Using a lysosome-containing (3500 x g) fraction, the release of lysosomal marker enzymes, acid phosphatase and aryl sulfatase, from lysosome which had been stimulated by L-ascorbic acid (AsA), was decreased dose-dependently, as was the inhibition of lipid peroxidation by PP in the fraction. Swelling of mitochondria induced by Fe2+ and AsA was also inhibited in the PP-injected rat. In microsomes, lipid peroxidation stimulated by AsA caused a decrease in activity of a microsomal marker enzyme, glucose 6-phosphatase, and in P450 content. The extent of the decrease by AsA, both in activity and content, was diminished in PP-administered rat liver microsomes. These results indicate that PP protects those subcellular fractions from deterioration by lipid peroxidation.