Autoxidized LA is classified into four groups, LA, LAHPO, SP and FP. Lysozyme is inactivated by these products in the increasing order as follows: FP less than LA less than LAHPO less than SP. The effects of these products on the amino acid composition of lysozyme is examined. All kinds of amino acid residues were not damaged until lysozyme was incubated with LA and LAHPO at 45 degrees C for 100 days. The susceptible amino acid residues attacked by the autoxidized products are tryptophan, lysine and histidine. The specific loss of methionine by SP occurs during acid-hydrolysis. The effect of SP was the strongest among the autoxidized products. FP was almost noneffective. The destructive actions of BP, MA and PA were compared with those of autoxidized products. Effects of these compounds did not resemble those of autoxidized products. It was concluded that tryptophan, lysine and histidine residues were specifically attacked by SP.