The product (9Z,12Z)-(11S)-11-hydroperoxyoctadeca-9,12-dienoate, is converted, more slowly, into (9Z,11E)-(13R)-13-hydroperoxyoctadeca-9,11-dienoate. The enzyme from the fungus Gaeumannomyces graminis requires Mn2+. It also acts on ¦Á-linolenate, whereas ¦Ã-linolenate is a poor substrate. Oleate and arachidonate are not substrates.