SUMMARYWe have used in situ hybridization (ISH ) and immunohistochemistry (IHC ) to investigate the kinetics of the expression for FceRI mRNA (a-, b-and c-chains), the a-chain protein product, as well as the phenotype of the mRNA-or protein-positive cells in allergen-induced late-phase skin reactions in atopic subjects. Compared with diluent controls, there were significant increases in the total numbers of mRNA+ cells for the a-, b-and c-chains for FceRI at all time-points (6, 24 and 48 hr) after allergen challenge (P<0·01). By double IHC/ISH significant increases in a-, band c-chain mRNA+ macrophages, eosinophils, mast cells and CD1a+ cells were also observed after allergen challenge (P<0·05). The distribution of FceRI subunit (a-, b-, or c-chain) mRNA+ co-localization was CD68+ macrophages (42-47%), EG2+ eosinophils (33-39%), tryptase+ mast cells (5-11%) and CD1a+ Langerhans' cells (2-4%). Using single IHC, significant increases in the total number of FceRI protein+ cells (P<0·01) were observed 24 and 48 hr after allergen challenge. Double IHC showed that the distribution of FceRI+ cells was tryptase+ mast cells (33%), CD68+ macrophages (36%), EG2+ eosinophils (20%), CD1a+ Langerhans' cells (4%) and unidentified cells (7%), at the 24-hr allergen-challenged sites. These observations suggest that the cutaneous late-phase reaction in man is associated with up-regulation of FceRI on eosinophils, macrophages, mast cells and Langerhans' cells.