Changes in the routine storage procedure of banked human blood, consisting of five days of thorough mixing of the blood suspension prior to measurement followed by a subsequent period of approximately 60 hours in which the cell suspension was allowed to settle, showed reversible increases in the number of microparticles during the periods of sedimentation. The cell aggregates were found to disaggregate during one day after thorough mixing of the cell suspension. The addition of a dipyridamole derivative (RA 233) reduced formation of microaggregates during the period of cell sedimentation. Besides the administration of additional stabilizing agents, a more frequent mixing of the cell suspensions during the storage period seems the easiest and most effective way to reduce microaggregate formation.