Human cord blood CD34(+)stem cells were allowed to differentiate in the presence of cytokines stem cell factor (SCF), granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumour necrosis factor alpha (TNF-alpha) into functional CD1a+dendritic cells (DC). A maximum of 1.9 x 10(6) CD1a+ cells were separated from the cells generated from 1.2 x 10(6) CD34(+) stem cells from an individual donor. The percentage of CD1a+cells separated rose to a maximum of 27% at day 11 and fell to 8% at 21 days. Reverse transcription-polymerase chain reaction analysis showed that interleukin 2 receptor, interleukin 3 receptor, interleukin 6 receptor, interleukin 12 receptor (IL-12R) and signal transducer and activator of transcription (STAT) 3, STAT 4 mRNA was expressed in all CD1a+cell populations throughout and appears to be constitutive. Expression of IL-12RmRNA was unexpected in CD1a+DC normally considered to be of myeloid lineage. Expression of interleukin 12 (IL-12) p40 subunit mRNA was not detected. Intermittent expression of the IL-12p35 subunit and IL-4R mRNA suggested that gene expression is inducible, but not obviously correlated with progressive DC development. Expression of mRNA for a spectrum of cytokine receptors indicates that CD1a+DC have the potential to respond to a variety of maturational signals.