In rat liver microsomes lipid peroxidation was found to be induced by chinoform-ferric chelate either in the presence or absence of NADPH, indicating the occurrence of both electron transfer-dependent and -independent peroxidation of microsomal lipids. In comparison with the case of ADP-ferric chelate, chinoform-ferric chelate gave a greater degree of electron transfer-independent lipid peroxidation. Either ferric ions or free chinoform induced little or no lipid peroxidation in the microsomes. Chinoform-ferric chelate-induced microsomal lipid peroxidation that leads to biomembrane damage was discussed in relation to the pathogenesis of subacute myelo-optico-neuropathy (SMON).