Antigens of the blood group systems Duffy, Kell and Kidd were found to be well developed during intrauterine life. Antigens k, Fy^a, Fy^b and Jk^b were first found in fetuses of 6-7 weeks gestation, antigens K and Jk^a at 10-11 weeks, and Kp^a at 16 weeks. Antigens Lu^a, Lu^b and Xg^a were considerably more poorly developed in the fetal samples than in the adult ones - both in frequency and in antigenic strength. Lu^a was found at the earliest in a fetus of 14 weeks gestation, and Lu^b in two of 10 weeks. The youngest Xg(a+) fetus was of 12 weeks gestation.