Twenty-nine murine monoclonal antibodies have been produced that react with shared idiotypes expressed by B-cell lymphomas and leukemias. We tested this panel of antibodies for reactivity with the paraproteins from 32 patients with multiple myeloma and 10 patients with monoclonal gammopathy of undetermined significance (MGUS). Thirteen of 42 paraproteins reacted with at least one antibody in this panel of anti- idiotypic antibodies. Six different anti-idiotypes demonstrated reactivity with the paraproteins. A similar frequency of reactivity was found for both myeloma and MGUS proteins. One antibody, S30–47, reacted with 6 of 32 (19%) of the paraproteins from patients with multiple myeloma, whereas this anti-idiotype only bound to 3% of non-Hodgkin's B- cell lymphomas and no cases of chronic lymphocytic leukemia. This anti- idiotype reacted with both components of a biphenotypic paraprotein (IgG kappa and IgG lambda) in one patient. In each of nine patients tested, plasma cells isolated from bone marrow were shown to be reactive with the same anti-idiotype we found to react with the paraprotein. Antishared idiotype antibodies may provide useful reagents for studies of patients with monoclonal gammopathies.