The immunocytochemical identification and characterization of indigenous dermal dendritic cells (dermal dendrocytes) using a rabbit polyclonal antibody to clotting enzyme factor XIII subunit A (FXIIIa) was carried out on normal and inflamed human cutaneous tissue. The immunophenotype of FXIIIa positive dendritic cells was analysed with a panel of 18 monoclonal antibodies using immunoperoxidase and double immunofluorescence staining techniques. The antibody against FXIIIa detected highly dendritic dermal cells located particularly in the upper reticular and papillary dermis. Double fluorescence microscopy showed that FXIIIa positive cells were bone marrow derived (HLe-I + ) and co-expressed monocyte, macrophage or antigen presenting cell markers (HLA-DR + , LFA-I + , HLA-DQ + , OKM5 + , MoI + , Mono-I + , Leu M3 + ). No labelling was obtained with cell markers for Langerhans cells (CDI), T lymphocytes (CD2), granulocytes (LeuMI) fibroblasts (Te7), intercellular adhesion molecule-I (ICAM-I) or endothelial cells (Factor VIII related antigen). Gamma interferon induced increased expression of HLA-DR and co-expression of ICAM-I on FXIIIa + dermal dendritic cells in normal skin in organ culture. Moreover, in benign inflammatory dermatoses such as atopic eczema and psoriasis there was an increased number of FXIIIa + , DR + , ICAM-I + cells in the upper dermis and foci of FXIIIa + cells in the epidermis closely associated with lymphocytes. FXIIIa positive cells in human skin represent a specific population of bone-marrow dermal dendritic cells, distinct from Langerhans cells, that share some features common to mononuclear phagocytes (monocyte/macrophages). In addition, the detection of HLA-DQ on 48% of FXIIIa + cells and the lack of OKMI in combination with high OKM5 expression suggests an antigen-presenting cell phenotype.