Systemic mastocytosis (SM) can be regarded as a tumorous proliferation of tissue mast cells (TMC) involving various organs, particularly the bone marrow. The infiltrates, however, are by no means composed exclusively of TMC, but also contain eosinophils and lymphocytes. The varying composition of the TMC infiltrates and the immunohistological characteristics of the lymphatic cells were the main subjects of investigation in this study. Three different types of bone marrow infiltrates could be identified: (1) A pure mastocytic infiltrate. (2) A mixed mastocytic/lymphocytic infiltrate. (3) A predominantly lymphocytic infiltrate containing loosely-scattered TMC. The mixed mastocytic/lymphocytic infiltrate seems to be a unique finding confined to the bone marrow in cases of SM, and is not detected in this conformation in other tissue sites normally involved in SM (spleen, liver, lymph nodes and skin), nor in cases of malignant mastocytosis. The lymphoid cells could be identified immunohistologically as being an admixture of T lymphocytes and B lymphocytes, while NK cells were virtually absent from the composite nodules. The TMC reacted strongly with antibodies (monoclonal or polyclonal) against vimentin, common leucocyte antigen, lysozyme, alpha 1-antichymotrypsin and alpha 1-antitrypsin, but were negative with a variety of other antibodies tested (UCHL1, MB1, Ki-B3, Leu-7, KL1, desmin, S-100 protein, F VIII-related antigen and chromogranin A).