In searching for the optimal glucose concentration, this lab has monitored ATP, 2,3-DPG, pH, and glucose levels of samples taken from full blood units stored for 6 weeks at 4 C. The blood was collected into CPD-adenine containing 100, 125, 150, 175, and 200 per cent of the glucose present in CPD. The units were stored as whole blood, soft packed (50 to 70% Hct), or hard packed units (80 to 95% Hct). ATP values in general did not decrease very greatly in whole blood units and only moderately in soft packed units. However, in hard packed units a steady progressive decrease in the ATP values was seen to begin at day 14. In these hard-packed units the only improvement with extra glucose was seen beginning at day 14 when ATP maintenance was better with 200 per cent glucose, but the improvement was not significant until day 42. However, at 35 days the ATP values for 200 and 175 per cent glucose were noticeably better than for the other preservatives. Therefore, it appears from this study that the glucose concentration in CPD-adenine for hard-packed cells should be at least 175 per cent of that in regularly formulated CPD. Also, there would appear to be an advantage of having 200 per cent glucose in those units of blood that may be stored beyond 35 days for emergency blood shortage times.