BACKGROUND Human red cells containing inositol hexaphosphate (IHP) have a lowered O2 affinity, though they are able to bind and carry about the same amount of oxygen as native cells. These modified cells therefore deliver oxygen more efficiently to the tissues, which is a property of potential clinical utility. Investigators set out to devise a system and procedure by which large volumes of IHP-containing red cells, suitable for transfusion, could be produced quickly and efficiently. METHODS The encapsulation of IHP into human red cells by use of several variations of static electroporation was performed to define the conditions necessary for optimal IHP incorporation and cell survival. These conditions were used as a starting point for optimization of a flow electroporation system. RESULTS When fresh human red cells in a 35 mM IHP solution are subjected to three exponential pulses of field strength of 2.98 +/- 0.064 kV per cm per pulse and pulse length of 2.0 +/- 0.2 msec per pulse while flowing through a cooled electroporation chamber, the condition of the resultant cells, according to the criteria used here, is optimized. After storage for 24 hours in plasma at 37 degrees C, the cells show more than 85-percent survival (in vitro) and hematologic indices nearly identical to those of unpulsed control cells. The p50 value of these cells, however, has doubled to 50.4 +/- 2.0 torr. The processing time for 1 unit of blood is 90 minutes. CONCLUSIONS These data indicate that the system described here can efficiently produce low-oxygen-affinity red cells in volumes that are useful in clinical applications.