The post-transfusion survival of multiple units of concentrated red cells was measured by ^51chromium automated differential agglutination and double 51chromium technics. Since the mean 24 h survivals were similar with both assay methods, these methods are considered acceptable in evaluating red cell preservation. Analysis of the paired data, however, revealed a significant difference in the results, indicating that an average of 3 % of ^51chromium eluted from red cells during the first 24 h.