An X ray fluorescence method for measurement of total serum phospholipid is described. A Folch extract of 1 ml of serum was evaporated and deposited on a paper disc by washing with chloroform. A coefficient of variation of 4% was obtained for samples containing at least 1 micromole of lipid, as phosphorus. The major advantage of the method is its speed (5 min/sample) in comparison to the phosphomolybdenum blue procedure. The method is free of interference from other lipid constituents and the phosphorous content can be standardized by comparison with very stable inorganic phosphates.