Dendritic cells (DCs) have been characterized as having an immature phenotype in infants when compared with adults; but it is unclear whether the phenotype or function of these populations changes during human intrauterine development. Three-colour flow cytometry was used to phenotype fetal/neonatal circulating DCs during the second half (>20-wk gestation) of pregnancy, (n = 34) and adults (n = 9). DCs were identified from peripheral blood mononuclear cells (PBMCs) or cord blood mononuclear cells (CBMCs) as staining brightly for HLA-DR but negative for T cell, B cell, monocyte, and NK cell lineage markers. The surface molecule of interest was detected in a third colour. During gestation CD34, a marker of immaturity was significantly higher, and CD4, a differentiation marker, was significantly lower than adult levels. The percentage of CD11c+ cells did not differ significantly at any age, although a trend to reduced intensity of expression at earlier stages of gestation was observed. Significantly fewer DCs expressed the IgG receptors CD32 and CD64 at all gestations. The percentage of HLA-DR+/lin- cells expressing CD40 was lowest at 20-23 wks and was always significantly lower on DCs from cord blood vs. adult blood. Similarly, the percentage of CD86+ and CD54+ DCs was significantly lower than adults throughout gestation. Thus, immaturity of cord blood DCs is likely to arise as a consequence of decreased ability to take up antigen (at least via IgG-mediated mechanisms) and reduced provision of co-stimulation.