In the former method for determining plasma free unsaturated fatty acids developed by Huh and Kim, it always showed relatively high values. The cause of high values was known to be a contamination of phospholipids in the extract of fatty acids. Some modifications were given. Plasma filtrate extracted with ordinary liped solvent was treated by chilled cetone to eliminate the interfering materials, phospholipids, followed by extraction of fatty acids with the solvent mixture. The defect of the former method could be completed by this modification giving sufficient results.