13L-hydroperoxy-9, 11-octadecadienoic acid was decomposed rapidly in the presence of hemoglobin. The product consisted of five major compounds, i.e. 13-keto-9, 11-octadecadienoic acid, 13L-hydroxy-9, 11-octadecadienoic acid, erythro-11-hydroxy-12, 13-epoxy-9-octadecenoic acid, threo-11-hydroxy-12, 13-epoxy-9-octadecenoic acid, and 9 DL-hydroxy-12, 13-epoxy-10-octadecenoic acid.