This study was made to determine the life span of adult red cells transfused to early preterm infants. Nineteen very preterm infants (birth weight, 878.7 +/- 221 g; gestational age, 26.8 +/- 1.5 wk at birth) were sampled weekly after their last blood transfusion to determine the level (%) of fetal Hb in their circulation. Two microliters of blood were subjected to reverse phase HPLC to separate the alpha, beta, and gamma globin components of their Hbs. The percent of fetal Hb (HbF) was calculated as gamma/gamma + beta x 100. The life span of the adult erythrocytes transfused was defined as the time interval between the transfusion and when the percentage of HbF in the recipient's circulation returns to the HbF levels that exist in the infant's autologous red cells (the maximum post transfusion HbF level). Twelve of the 19 infants were followed until their autologous HbF levels were reached. Their mean adult red blood cell life span was 56.4 +/- 7 d (range: 46-68 d). The results obtained in this study imply that the number of days after a transfusion at which half the cells infused remain in the circulation in a preterm infant is about 30 d.