Abstract.The lecithin:cholesterol acyltransferase (LCAT) activity and lysolecithin content of human blood stored under standard blood transfusion service conditions at 40C for 6 weeks has been investigated. Cooling the blood to 40C rapidly inactivates the LCAT reaction, but the enzyme is not denatured during storage under these conditions. Citrate in the anticoagulant did not activate the LCAT reaction in freshly‐taken whole blood. The total phospholipid and total lysolecithin content of whole blood decreased during storage at 40C for 6 weeks. The lysolecithin content of fresh red cells (2.0–3.0 αmol lysolecithin × 10‐11per cell) showed no significant change during the storage