A series of six derivatives of arachidonic acid containing a conjugated triene system and hydroxyl groups at C-15 and at C-8 or C-14 (15-series leukotrienes) was isolated from eosinophil-rich human leukocyte preparations. Among classes of leukocytes in these preparations, eosinophils were the dominant source of the 15-series leukotrienes. The most abundant arachidonate metabolite recovered from human eosinophils was 15(S)-hydroxy-5,8,11-cis-13-trans-eicosatetraenoic acid. This compound was synthesized by a lipoxygenase enzyme and is a reduction product of the precursor to the 15-series leukotrienes (15(S)-hydroperoxy-5,8,11-cis-13-trans-eicosatetraenoic acid).