The effect of protoporphyrin (PP) on membrane fluidity was investigated by electron paramagnetic resonance spectroscopy using doxyl stearate spin probes in relation to the anti-lipidperoxidative effect of PP. PP decreased the membrane fluidity in rat liver microsomes at concentrations above 1 mM and also in phosphatidylcholine (PC)-choesterol (Cho) (100 : 8, a molar basis) liposomes. The lipid peroxidation stimulated by Fe²⁺ and L-ascorbic acid in those membrane preparations was attenuated along with the decrease in membrane fluidity by PP. Similar results were also found in Cho-rich PC (100 : 30 to 100) liposomes having less fluidity. These results suggest that the decrease in the membrane fluidity caused by PP may be involved in the antioxidative action of PP.