To evaluate the possibility that changes in lipid composition might be related to the functional lesion that develops when platelets are stored as concentrates for several days, we measured lipid constituents of platelets in freshly prepared concentrates and in concentrates stored for 72 hr at 4 degrees C or at 20 degrees C under standard blood banking conditions. At 20 degrees C, but not at 4 degrees C, platelets lost about 15% of total cholesterol and 7%--11% of total phospholipid. The distribution of individual phospholipids remained unchanged. This was also true of the fatty acid distribution in total phospholipids and in individual phospholipids except for a statistically significant reduction of linoleic acid (18:2) and an increase in oleic acid (18:1) in phosphatidyl inositol (PI). Platelets collected in citrate-phosphate- dextrose (CPD) anticoagulant did not differ significantly in lipid composition from those collected in acid-citrate-dextrose (ACD) anticoagulant during the period of observation. These findings do not provide a basis to suggest that functional abnormalities developing in stored platelets are related to changes in lipid composition.