During the NADPH-Fe-induced peroxidation of liver microsomal lipids products are formed which are provided with cytopathological activities. In a previous study one of the major products was identified as an aldehyde of the 4-hydroxyalkenal class, namely 4-hydroxynonenal. In the present study another cytotoxic product has been isolated and identified as 4,5-dihydroxy-2,3-decenal. The isolation was performed by means of thin-layer chromatography and high-pressure liquid chromatography and the structure was ascertained mainly by means of mass spectroscopy of the free aldehyde and of its derivatives. In the absence of NADPH-Fe liver microsomes produced no 4,5-dihydroxydecenal. The inhibitory activity of 4,5-dihydroxydecenal on microsomal glucose-6-phosphatase is somewhat lower than that exhibited by 4-hydroxynonenal. This lower inhibitory activity correlates with the lower capacity to bind to the microsomal protein of 4,5-dihydroxydecenal as compared to 4-hydroxynonenal. The reactivities of the two aldehydes with cysteine were comparable. The production of toxic aldehydes may represent a mechanism by which lipid peroxidation causes deleterious effects on cellular functions.