Bone marrow aspirates from 10 patients with multiple myeloma (6 IgG myeloma, 3 IgA myeloma, 1 Bence Jones myeloma) were examined for nucleolus-associated localization of J chains. On light microscopy using the PAP method, almost all or a significant number of myeloma cells exhibited intranuclear spots stained with anti-J chains in 3 (IgG myeloma) out of the 8 cases positive for cytoplasmic J chains. In contrast, the nucleus of myeloma cells examined was constantly negative for anti-heavy and -light chains. Immunoelectron microscopically, J chain was identified as dicrete round electron-dense precipitates, corresponding to the whole nucleolus, and as sparsely distributed, small electron-dense deposits in the nucleus. In addition, some connections were found between nuclear and nucleolar electron-dense precipitates. Several possible explanations have been proposed to account for this localization of J chains.