We have studied the fatty acid composition of total lipids of erythrocytes and vesicles either during storage of human erythrocytes in their own plasma under blood bank conditions or during incubation at 37 degrees C in protein-free media. Vesicles appear as a heterogeneous population with a diameter of about 130 nm and a varying content of hemoglobin. The fatty acid pattern of vesicle total lipids changes in respect to control erythrocytes, while in erythrocytes it remains fairly constant during the total ageing period. Our results indicate a marked rearrangement of the membrane components during blood storage.