Acetone extracts of Euglena gracilis Klebs var. bacillaris Cori contain several lipoxygenase and cyclooxygenase products as measured by radioimmuno-assay. Three times more of the serologically active products are found in cells grown in the dark than in cells grown in the light. Lack of coincidence of their retention times on high performance liquid chromatography with authentic oxygenation products of arachidonic acid suggest that most of the serologically active compounds are derivatives of polyenoic fatty acids other than arachidonic acid. Several fractions react with anti-12-hydroxyarachidonic acid (an antiserum that recognizes mono- and di-hydroxyarachidonic acids). Three of these fractions have UV absorption spectra characteristic of conjugated trienes.