Earlier this laboratory recognized lipoxygenase catalyzed reactions as a novel pathway for xenobiotic metabolism. To further explore the spectrum of reactions catalyzed by lipoxygenase, sulfoxidation of thiobenzamide was studied. Purified soybean lipoxygenase was found to oxidize thiobenzamide to thiobenzamide sulfoxide in the presence of linoleic acid at a rate of 241 nmoles/min/nmole enzyme. The reaction was dependent upon enzyme, pH, thiobenzamide and linoleic acid concentration. Other polyunsaturated fatty acids namely arachidonic acid, cis, 11, 14-eicosadienoic acid and linolenic acid also supported the sulfoxidation reaction. Nordihydroguaiaretic acid and phenidone, the classical inhibitors of lipoxygenase, significantly blocked the sulfoxidation of thiobenzamide.