The study by W. G. Niehaus, Jr., and B. Samuelsson identifies malonaldehyde as the thiobarbituric acid-reacting material produced during enzymatic microsomal lipid peroxidation. The malonaldehyde was condensed with urea to form 2-hydroxy-pyrimidine, which was characterized by its ultraviolet spectrum, chromatographic properties, and mass spectrum. Incubations with phosphatidyl choline labeled with tritiated arachidonate yielded 2-hydroxy-pyrimidine with a specific activity nearly equal to that of the phospholipid arachidonate, confirming that phospholipid arachidonate is the major source of malonaldehyde. Incubations with tritiated arachidonic acid yielded 2-hydroxy-pyrimidine with a specific activity nearly two orders of magnitude less, indicating that free arachidonic acid is not an obligatory intermediate in the production of malonaldehyde. The results suggest that most malonaldehyde formed during microsomal lipid peroxidation arises directly from phospholipid arachidonate, with a minor contribution from phospholipid docosahexaenoate. The study also highlights the complexity of the reaction involving liver microsomes, NADPH, and oxygen, which produces multiple radioactive products.The study by W. G. Niehaus, Jr., and B. Samuelsson identifies malonaldehyde as the thiobarbituric acid-reacting material produced during enzymatic microsomal lipid peroxidation. The malonaldehyde was condensed with urea to form 2-hydroxy-pyrimidine, which was characterized by its ultraviolet spectrum, chromatographic properties, and mass spectrum. Incubations with phosphatidyl choline labeled with tritiated arachidonate yielded 2-hydroxy-pyrimidine with a specific activity nearly equal to that of the phospholipid arachidonate, confirming that phospholipid arachidonate is the major source of malonaldehyde. Incubations with tritiated arachidonic acid yielded 2-hydroxy-pyrimidine with a specific activity nearly two orders of magnitude less, indicating that free arachidonic acid is not an obligatory intermediate in the production of malonaldehyde. The results suggest that most malonaldehyde formed during microsomal lipid peroxidation arises directly from phospholipid arachidonate, with a minor contribution from phospholipid docosahexaenoate. The study also highlights the complexity of the reaction involving liver microsomes, NADPH, and oxygen, which produces multiple radioactive products.