When whole blood is supplemented with adenine, the erythrocytes are suitable for transfusion after at least 35 days of storage at 4C, but information is not available as to whether such cells remain satisfactory for blood typing and pretransfusion tests for evidence of incompatibility. Accordingly, we designed experiments to evaluate by AutoAnalyzer the specific agglutinability of erythrocytes obtained from whole blood stored at 4C. Seven normal volunteer donors of both sexes, aged between 22 and 31 years, were chosen and bled periodically so that, over a threeday testing period, all blood specimens from each donor could be evaluated. Each donor provided at each bleeding s d i a e n t blood to permit aliquots of 20 ml to be stored as clotted blood and 8 ml to be stored as citrated whole blood. Four kinds of citrate solution were used: ACD Formula A, CPD, ACD sup plemented with adenine, and CPD supplemented with adenine. All clotted specimens were tested after 0, 1, 2, 3, 4, and 5 weeks of storage, while all citrated specimens were tested after 0, 1, 2, 3, 4, 5, 6, 8, and 10 weeks of storage. S i x blood group systems were used for evaluation with the following reagents: anti-A or anti-B, anti-Rhl (Rho or D), anti-KZ (k or Cellano), anti-Jka, anti-Fya Loss of specific hemagglutinability of stored cells was not due to loss of specific antigenic sites because stored cells were as effective as fresh cells for the absorption of anti-B, anti-Rhl, and anti-M. Loss might, however, be related to an increase in the accumulation of 1 6 , fibrinogen, and albumin at the surfaces of erythrocytes.