The RelB gene product is a member of the nuclear factor (NF)‐κB family of transcription factors. It has been identified recently within mouse antigen‐presenting cells and human monocyte‐derived dendritic cells (DC). Disruption of the mouse RelB gene is accompanied, amongst other phenotypes, by abnormalities in the antigen‐presenting cell lineages. In order to define RelB expression during human DC differentiation, we have analysed RelB mRNA by reverse transcriptase–polymerase chain reaction and RelB protein by intracellular staining in CD34⁺ precursors and different types of DC preparations. RelB mRNA was not detected in CD34⁺ precursor populations. Fresh blood DC (lineage⁻human leucocyte antigen‐DR⁺ (lin⁻HLA‐DR⁺)) lacked RelB mRNA and cytoplasmic RelB protein but a period of in vitro culture induced RelB expression in blood DC. Purified Langerhans’ cells (LC) (CD1a⁺ HLA‐DR⁺) failed to express RelB mRNA. Immunocytochemical staining identified RelB protein in human skin epithelium. RelB protein was expressed in a very few CD1a⁺, CD83⁺ or CMRF‐44⁺ dermal DC but was not present in CD1a⁺ LC. Tonsil DC (lin⁻HLA‐DR⁺ CMRF‐44⁺) were positive for RelB mRNA and RelB protein. Intestinal DC (HLA‐DR⁺) also lacked immunoreactive RelB protein. The majority of interdigitating CD83⁺, CMRF‐44⁺, CMRF‐56⁺ or p55⁺ DC located in paracortical T‐lymphocyte areas of lymph node and tonsil contained RelB protein. The expression of RelB mRNA and RelB protein correlates with the activated phase of blood DC and the postmigration cell (activated) stage of tissue DC development.