Ultrastructural studies of intranuclear inclusions in plasma cells and lymphocytes from patients with different forms of monoclonal gammopathies are presented. The inclusions occurred as two distinct morphologic and ultrastructural types. In three patients, two with IgA myeloma and one with Waldenström's macroglobulinemia, the inclusions were variably PAS-positive in routine sections and composed of electron-light amorphous material ultrastructurally. In one patient with an IgG monoclonal gammopathy, the plasma cells contained inclusions that were PAS negative by light microscopy and primarily osmiophilic ultrastructurally. One patient with an immunoblastic sarcoma and an IgM monoclonal had plasma cells with numerous inclusions of both types. There was a distinct spatial relationship between the electron-light amorphous material and the osmiophilic deposits in many of the inclusions in cells from this patient. In cells from two patients in which osmiophilic inclusions were found, they were present in the cytoplasm as well as the nucleus. The intranuclear inclusions appeared to develop from accumulation of material in the perinuclear cisterna, resulting in single or multiple invaginations of the nuclear structure.