Neoplastic cells from 9 patients affected with a "histiocytic" lymphoma were studied with 5 membrane markers of B or T lymphocytes. In 2 patients a monoclonal B-cell proliferation was found; they had been affected previously with well documented B-cell proliferations: chronic lymphocytic leukemia or Waldenström's macroglobulinemia. The blast cells of 2 other patients had T-cell features; in a fifth case, the abnormal cells carried only a strong receptor for the Fc fragment of IgG, which suggested their truly monocytic origin. In 4 patients, the cells had no detectable surface markers. These findings demonstrated that this group of lymphomas is heterogenous, that the term "histiocytic" appears to be wrong in most instances, and that the cellular origin of the malignant cells frequently remains unidentified and thus prevents a satisfactory new classification.