Linoleic acid, the predominant polyunsaturated fatty acid in the diet, can be metabolized by cyclooxygenase, lipoxygenase and P450 enzymes. The monohydroxy lipoxygenation products of linoleic acid, 9- and 13-hydroxyoctadecadienoic acids (9(S)- and 13(S)-HODEs), are the most widely distributed of the known linoleic acid metabolites. These compounds exhibit interesting biological activities, including regulation of platelet function, maintenance of vascular thromboresistance and transduction of the cellular responses to certain growth factors. In view of their biological significance, we have produced polyclonal antibodies for the first time to these bioactive lipids to develop an easy, inexpensive, sensitive, specific and rapid enzyme immunoassay method for these bioactive lipids.