Rabbit reticulocytes metabolize exogenous polyenoic fatty acids via three different pathways. (i) incorporation into the cellular ester lipids, predominantly into phospholipids, (ii) beta-oxidation forming CO2 and (iii) lipoxygenase reaction. The lipoxygenase pathway contributes to about 30% to the metabolism of exogenously added linoleic acid. The endogenous substrates of the lipoxygenase pathway are not only the free polyenoic fatty acids bound to the cellular membranes but also the membrane phospholipids. The lipoxygenase products detected in the membrane lipids have been isolated and their complete chemical structure has been identified as 13-hydroxy-9Z,11E-octadecadienoic acid. 9-hydroxy-10E,12Z-octadecadienoic acid, their all E isomers and 15-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid. Subcellular fractionation of different cellular membranes indicated that these products occur in both the mitochondrial membranes and the plasma membrane. By quantitative HPLC analysis it has been shown that the mitochondrial membranes contain about 3 times more oxygenated fatty acids than the plasma membranes; one out of ten linoleic acid residues in the mitochondrial membranes is present as hydroxylated derivative. The pattern of lipoxygenase products detected in the mitochondrial membranes was much more unspecific than that of plasma membranes. These data are discussed in the light of the involvement of the lipoxygenase pathway in the degradation of mitochondria during the maturation of red blood cells.