The lower limit for the volume of whole blood that may be collected for transfusion into standard blood recipient sets is 405 ml. Each year it is estimated that 82,500 to 161,700 units are drawn which contain between 275 and 405 ml. This study evaluated whether these "undercollected" units would be suitable for transfusion. Twenty normal adults donated both a "standard" unit (450 ml) and an "undercollected" unit (275 ml) in 63 ml of citrate-phosphate-dextrose-adenine-one (CPDA-1). The units were packed within 4 hours (mean Hct 71%) and stored undisturbed at 4 degrees C for 35 days. Aliquots of 1 to 2 ml of red cells from each unit were then labeled with 51chromium (51Cr) and reinfused into the original donor. The mean 24-hour survival of the 450-ml units was 78.8 percent (SD 12.2, SEM 2.7), while the mean 24-hour survival of the 275-ml units was 87.7 percent (SD 10.7, SEM 2.4; p less than .01). Seven inadvertently undercollected units (mean vol: 295 ml) had a mean 24-hour survival of 91.9 percent. The higher concentration of dextrose and adenine in the undercollected units may improve posttransfusion red cell viability. These data suggest that 275 ml is the minimum acceptable volume for blood donated into CPDA-1.