The objective of this study was to compare the expression of primitive cell-surface antigens on CD34+ cells from early in gestation to those from term gestations. Fetal blood samples were obtained from 10 early gestation (21.0 ± 0.8 [SE] weeks) and 12 term gestation (39.3 ± 0.4 weeks) fetuses. The mononuclear cell population was separated by red cell lysis. Two-color flow cytometry was used to assess cell surface antigen coexpression of CD34 with CD33, CD38, and HLA-DR as well as staining by a cocktail of monoclonal antibodies for lineage-associated (Lin) antigens (CD2, CD10, CD11b, CD19, CD20, CD33, CD36, 7B9, and Glycophorin-A). The frequency of CD34+ cells (5.5 ± 0.9 versus 1.5 ± 0.2, p < 0.001) was significantly higher in the early gestational age group. Within the CD34+ population, the frequency of CD34+/CD38− cells (81.8 ± 9.9 versus 51.3 ± 7.7, p = 0.02) and CD34+/DR− cells (15.3 ± 7.4 versus 8.2 ± 2.7, p = 0.05) was also higher in the early gestational age group. In contrast, CD34+/CD33− (51.8 ± 10.1 versus 83.0 ± 6.1, p = 0.02) and CD34+/Lin− cells (15.9 ± 7.0 versus 51.8 ± 6.9, p < 0.01) were higher in the term gestation group. The high percentage of CD34+, CD34+/CD38−, and CD34+/DR− cells supports our hypothesis that early gestational age fetal blood has a higher frequency of primitive hematopoietic progenitor/stem cells than does umbilical cord blood at term. This suggests that hematopoietic progenitor/stem cells in early fetal blood may be a desirable target for in utero gene therapy. However, further studies to characterize the functional properties of CD34+ cell subsets at different stages of fetal development will be necessary to determine the appropriateness of targeting fetal hematopoietic cells for in utero gene therapy. The higher frequency of CD34+/CD33− and CD34+/Lin− cells from term gestational age fetuses was unexpected, and the significance of this finding is unclear at this time.