The progressive appearance of erythrocyte antigenic activity in plasma from donations of human blood taken into acid citrate-dextrose (ACD) and stored at 4 degrees D for transfusion has been studied. It has been shown by specific inhibition tests that A, B, H and P1 activities are associated with membrane-bounded microvesicles which are released from the red cell membrane during the disc-echinocyte-spherocyte shape transformation. Although M group activity appears to be lost from the red cell membrane during storage, significant recovery on the microvesicles is not obtained even though these seem to contain the M and N group intrinsic protein, glycophorin.