Use of monoclonal antibody KP1 for identifying normal and neoplastic human mast cells. The monoclonal antibody KP1 (CD68) was used to stain normal and neoplastic monocytes and macrophages in routinely processed, paraffin wax embedded tissue: mast cells also exhibited strong, consistent cytoplasmic immunoreactivity. Light microscopic findings were corroborated by electron microscopical and immunocytochemical findings. The predominant sites of immunoreactivity were the specific intracytoplasmic granules of the mast cells. All mast cell subtypes--that is, normal and reactive mast cells, such as those in lymph nodes exhibiting chronic non-specific lymphadenitis, and malignant or neoplastic mast cells in various types of mastocytosis--reacted with this antibody. This finding is of diagnostic importance, because mast cell proliferation could be mistaken for histiocyte proliferation. It also supports the hypothesis that mast cells derive from the bone marrow. 