Bone marrow specimens from 363 patients who had monoclonal gammopathies were studied; 192 patients (52%) had multiple myeloma, one had a solitary plasmacytoma, 68 (19%) had lymphoproliferative disorders, 65 (18%) had monoclonal gammopathies of undetermined significance (MGUS), 24 (7%) had nonimmunocyte malignancies, seven (2%) had amyloidosis, and six (2%) had autoimmune diseases. For patients with multiple myeloma, the monoclonal immunoglobulin (MIg) was IgG in 64%, IgA in 25%, IgD in 2%; 8% had light chain disease and 1% had biclonal spikes; none had an IgM MIg. The multiple myelomas were divided into five morphologic types, but the cytologic characteristic could be correlated with immunoglobulin class in only a very small number of cases of IgA multiple myeloma. Of patients with lymphoproliferative diseases, 59% had an IgM MIg, 32% had IgG, 6% had IgA, and 3% were biclonal. The lymphoproliferative disorders associated with a g were primarily small lymphocyte (B) malignancies (38%), plasmacytoid lymphomas (21%), and follicular center cell lymphomas (25%). The clinical findings of Waldenstrom's macroglobulinemia were present in 17 of the cases of lymphoproliferative disorder (nine plasmacytoid lymphomas and eight small lymphocyte (B) lymphomas); all had IgM MIg. Nuclear inclusions were found in neoplastic cells in 32% of 40 cases of lymphoproliferative disorders with an IgM MIg, but were also found in 19% of cases of IgA multiple myeloma. Of the patients with MGUS, 68% had an IgG MIG, 14% had IgA, 15% IgM, and 3% were biclonal; there was no unifying clinical or morphologic feature among this group. Less than 2g/dl of MIg was found in 24% of multiple myelomas, 61% of lymphoproliferative disorders, and 71% of MGUS. Nonmonoclonal immunoglobulins were decreased in 83% of multiple myelomas, 46% of lymphoproliferative disorders, and 29% of MGUS. Bence Jones protein was found in 69% of patients with multiple myeloma and 26% of patients with MGUS. Of eight patients in the study who were younger than 20 years old, two had lymphoproliferative disorders, two had monoblastic leukemia, and four had aplastic anemia; in three of the latter, the MIg appeared following bone marrow transplant.