14C cyanate as a tag for red cell survival in normal and uremic man

14C cyanate was employed as a red cell tag. Incubated cells were washed before reinjection to remove free cyanate. Neither early or late elution was observed, and the mean red cell life span of 10 normal subjects was 115+/-12 days. In nine uremic subjects the mean life span corrected for blood loss was 73 days with a mean red cell turnover of about 0.79 times basal. Survival curves in normal subjects were similar to those observed with DF32P and in uremic subjects to those with 51Cr corrected for elution. These observatios show the reliability of 14C cyanate as a nonelutable red cell tag.