Factor XIIIa has previously been used to identify a subgroup of dermal dendritic cells which co-express class II MHC antigens, intercellular adhesion molecule-1 (ICAM-1) and lymphocyte function associated antigen-1 (LFA-1), leading to the suggestion that they play a central role in skin immune responses. To examine whether these cells may also be important constituents of other organs we have performed an immunohistochemical survey of normal tissue to identify the distribution of this factor XIIIa positive subgroup of dendritic cells in humans. Routine tissue sections from a range of organs were assayed for factor XIIIa expression using a standard peroxidase-antiperoxidase (PAP) immunolabelling technique. Dendritic, factor XIIIa positive cells were observed in all tissues studied, but were most numerous in skin and mucosal tissues (gastrointestinal tract, bladder). They were also observed associated with epithelial structures in lung and kidney, but were only rarely observed in liver, thyroid, testis and spleen. The distinctive distribution of these cells is consistent with an important role for them in immune responses at those sites.