We report the establishment of seven mouse-mouse hybridoma cell lines secreting monoclonal antibodies with specificity for granule components of all human mast cells. Reactivity is directed against a molecule which is also found intracytoplasmically in human mature small intestinal enterocytes, liver parenchymal cells, and kidney proximal tubule epithelial cells. No reactivity of these antibodies was found with any other human or animal cell type examined. In particular, the antibodies did not react with basophils or other haemopoietic cell types. This study shows the potential of specific monoclonal antibodies as a tool for identifying and enumerating infiltrating mast cells in tissues. Such antibodies should be of value in investigations into the role of the mast cell in immunological reactions and hypersensitivity diseases.