Our aim was to evaluate the number of progenitor cells circulating in an α-thalassemic fetus during its infusion in utero with paternal CD34+ and adult red cells and to compare those values with those circulating in normal and non-thalassemic anemic fetuses of matched gestational age. The treatment of the α-thalassemic fetus has been described elsewhere. Fetal blood was obtained from normal and anemic fetuses by fetal blood sampling for diagnostic or therapeutic purposes according to a protocol approved by the human subject committee. The number of progenitor cells in fetal blood was estimated on the basis of the number of colonies they gave rise to in semisolid cultures. The α-thalassemic fetus, as did the other fetuses analyzed, contained high numbers (10 6–10 7 depending on the age) of progenitor cells, values which were higher than the number (10 4–10 5) of paternal progenitor cells being transplanted. Progenitor cells with adult characteristics (adult kinetics of differentiation) were detected rapidly (10 min) after the CD34+ cell infusion, but were not detectable 2–3 weeks after the transplant. These results indicate that adult progenitor cells do not have a numerical advantage when transplanted into α-thalassemic fetuses.