AbstractThis review covers studies on membrane‐bound phospholipid desaturases in yeast and rat liver carried out in this laboratory. In yeast the desaturase system was shown to effect the direct desaturation of dioleoyl‐lecithin to dilinoleoyl‐lecithin. In rat liver the desaturase was capable of converting 2‐eicosatrienoyl‐lecithin to 2‐arachidonoyl‐lecithin. Both systems required reduced pyridine nucleotides, O2 and cytochrome b5. Eicosatrienoyl‐lecithin desaturase along with eicosatrienoyl‐CoA desaturase of rat liver microsomes was solubilized with detergents and purified 7–8‐fold from the microsomal pellets. Both activities were reconstituted in the presence of deoxycholate on addition of the other components of the cytochrome b5‐electron transport chain (cytochrome b5 and NADH‐cytochrome b5 reductase) to the solubilized desaturase; addition of lecithin further stimulated the activities. The demonstration of desaturation of eicosatrienoyl‐lecithin by a solubilized and partially purified desaturase provides strong evidence for the direct desaturation of the lecithin substrate without prior conversion to the acyl‐CoA thiolester.