SummaryThe survival of Cr51‐labelled placental red cells of 10 full‐term and 6 premature infants was studied after transfusion to adult recpients. The mean apparent half‐life of the red cells from full‐term infants was found to be 22.8 days, and that of red cells from premature infants 15.8 days (cf. 27.5 days for red cells from adults). Results obtained by others using the differential agglutination method have indicated a longer life span than that corresponding to the Cr51half‐times above mentioned. The most probable explanation of this difference seems to be that Cr51is eluted more rapidly from fetal red cells than from red cells of adults. This hypothesis is corroborated by the results of experiments in vitro on intact