Effect of the soluble fraction of rat liver on lipid peroxidation in hepatic microsomes was investigated. The soluble fraction markedly accelerated the lipid peroxidation in microsomes, even in the presence of 40μM Fe²⁺ and/or 10μM L-ascorbic acid. However, the soluble fraction effectively inhibited the NADPH-dependent lipid peroxidation in microsomes, especially in the presence of both glucose-6-phosphate and glucose-6-phosphate dehydrogenase. The inhibitory effect on the microsomal lipid peroxidation induced by NADPH-regenerating system was reduced by heating the soluble fraction. The addition of Zn²⁺, an inhibitor of glutathione reductase, to the reaction mixture weakened the anti-peroxidative action of the soluble fraction on the lipid peroxide formation in microsomes together with NADPH-regenerating system and, inversely, oxidized glutathione enhanced the action of this fraction. In addition, presence of oxidized glutathione and glutathione reductase preparation was as effective as reduced glutathione in depressing lipid peroxidation in microsomes stimulated by NADPH-regenerating system. These findings suggest that both the activation of glutathione reductase system by NADPH and a heat-stable factor (s) take part in the inhibitory effect of the soluble fraction on the NADPH-linked lipid peroxidation in microsomes. This soluble fraction also inhibited the microsomal lipid peroxidation stimulated by L-ascorbic acid at the concentration of 50μM or above, and the inhibitory effect was slightly increased by heating of this fraction. It is assumed that the inhibitory effect of the soluble fraction on L-ascorbic acid-dependent lipid peroxidation might be due to the mechanism different from that acts on the NADPH-dependent lipid peroxidation.