The preservation of [human] red cells hard packed to a hematocrit of over 80% from blood collected in citrate-phosphate-dextrose (CPD) or CPD-adenine (CPDA-1) was investigated. After 21 days of storage, cells that were collected in CPD solution had consumed most or all of the available glucose and manifested markedly impaired viability after reinfusion into the normal donor. Red cells prepared from blood collected in CPDA-1, a medium containing supplementary adenine and an increased amount of glucose, maintained higher glucose and ATP levels and, in most instances, manifested satisfactory posttransfusion viability. CPDA-1 provides a longer shelf life of stored blood and a better hard-packed red cell concentrate for transfusion at 21 days.