A two-enzyme system involving a lipase from a Pseudomonas sp. and an extract of potato tubers containing lipoxygenase was used to convert triacylglycerols to 9-hydroperoxy fatty acids. Highest yields (up to 25%) were obtained with 1 to 20 g l(-1) trilinolein as substrate after 5 h under O-2 at pH 6 and 25 degreesC. The product structure was confirmed by H-1 NMR, MS and IR.