Phosphatidylcholines (PCs) bearing various kinds of aldehydic acyl chains at the sn-2 position have been detected in atherosclerotic tissues. However, 1-acyl-2-(13'-oxo-9',11'-tridecadienoyl)- sn-glycero-3-phosphocholine and other alpha;beta;&gama;σ-unsaturated aldehyde PCs have not. To determine whether this might be due to their high chemical reactivity with biomolecules, we investigated the reactions of 1-stearoyl-2-(13'-oxo-9',11'-tridecadienoyl)- sn-glycero-3-phosphocholine (OTDA-PC, where OTDA refers to the oxo-tridecadienoyl moiety) with nucleophilic amino acids and peptides by means of electrospray mass spectroscopy. OTDA-PC formed Michael adducts with lysine, arginine, histidine, hippuryl lysine and hippuryl arginine, but was surprisingly unreactive with cysteine or glutathione. When 1-stearoyl-2-(13'-hydroperoxy-9'Z,11'E,15'Z-octadecatrienoyl)- sn-glycero-3-phosphocholine (PC-LNA-OOH, where LNA-OOH denotes the linolenic acid hydroperoxide moiety)　was decomposed in the presence of the reactive lysine, OTDA-PC was still detected as a major product. However, OTDA-PC could not be detected when 1-stearoyl-2-(13'-hydroperoxy-9'Z,11'E-octadecadienoyl)- sn-glycero-3-phosphocholine (PC-LA-OOH, where LA-OOH refers to linoleic acid hydroperoxide) was decomposed in the presence or absence of lysine. Since linoleic acid is the major polyunsaturated fatty acid in atherosclerotic tissues, these results indicate that formation of OTDA-PC in only minor amounts in such tissues may explain its not having been detected in them. Surprisingly, 1-stearoyl-2-(9'-oxononanoyl)- sn-glycero-3-phosphocholine was the major aldehydic product of the decomposition of PC-LA-OOH under anaerobic conditions.