The yolk sac and aorto-gonad-mesonephros region are well recognized as the principal sites of hematopoiesis in the developing embryo, and the liver is the principal site of hematopoiesis in the fetus. However, little is known about circulating hematopoietic stem and progenitor cells in early fetal life. We investigated the number and characteristics of circulating progenitors in first trimester blood of 64 human fetuses (median gestational age, 10(+4) weeks; range, 7(+6)-13(+6) weeks). CD34+ cells accounted for 5.1 +/- 1.0% of CD45+ cells in first trimester blood, which is significantly more than in term cord blood (0.4 +/- 0.03%; P =.0015). However, the concentration of CD34+ cells (6.6 +/- 2.4 x 10(4)/mL) was similar to that in term cord blood (5.6 +/- 3.9 x 10(4)/mL). The total number of progenitors cultured from unsorted mononuclear cells (MNCs) in first trimester blood was 19.2 +/- 2.1 x 10(3)/mL, which is similar to that in term cord blood (26.4 +/- 5.6 x 10(3)/mL). All lineages were seen: colony-forming unit-GEMM (CFU-GEMM), CFU-GM, BFU-e, BFU-MK, and CFU-MK. Clonogenic assays of CD34+ cells purified from first trimester samples produced mainly two lineages: BFU-e (39.0 +/- 9.6 x 10(3)/mL CD34+ cells) and CFU-GEMM (22.6 +/- 4.7 x 10(3)/mL CD34+ cells). Short-term liquid culture of first trimester blood MNCs in SCF + IL-3 + Flt-3 (stem cell factor + interleukin-3 + Flt-3) increased, by 7-fold, the numbers of CFU-GEMM and induced a dramatic increase in BFU-e (65.6 +/- 12.1-fold). These data show that significant numbers of committed and multipotent progenitors with capacity for expansion circulate in first trimester fetal blood and can be CD34 selected. These cells should be suitable targets for gene transfer and stem cell transplantation and, because fetal hematopoietic progenitors have been demonstrated in the maternal circulation from early gestation, may also be manipulated for noninvasive prenatal diagnosis of major genetic disorders. (Blood. 2000;95:1967-1972)