A symposium was centered around the unsaturated phosphatidyl choline molecule and organized in order to assemble and coordi nate theoretical views with facts and results. The presence of a high percentage of essential fatty acids in unsaturated phos phatidylcholine gave rise to the essential phospholipid concept. An overview of the biological significance of phospholipids and a review of a specific phosphatidylcholine-related enzyme, namely LCAT or lecithin cholesterol acyl transferase, open these proceedings. The simultaneous use of the synonyms - leci thin and phosphatidylcholine - was...