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When liver microsomes are incubated in the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH), their constituent lipids undergo peroxidative degeneration. If erythrocytes are present in such a peroxidizing system, they hemolyze. Filtrates obtained by ultrafiltration of peroxidizing microsomal systems were found to have the capacity to produce prelytic damage in red cells. Filtrates obtained from microsomes that had not undergone peroxidative lipid decomposition were inert. The toxic activity in the active filtrates was not due to continuing oxidation of NADPH nor to continuing liver microsomal lipid peroxidation. Neither the chemical identity of the toxic product or products in active filtrates nor the mechanisms involved in the erythrocyte damage are known at this time.

Rat liver microsomes prepared by a calcium aggregation method have been found capable of readily undergoing NADPH induced lipid peroxidation. Malonic dialdehyde production of calcium bound microsomes is equivalent to that attained by microsomes prepared by ultracentrifugation. In addition these calcium bound microsomes can be quickly removed from suspension by means of ultrafiltration.