A dual expression of IgM and IgG was found in the cytoplasm of nearly all plasma cells in the bone marrow from a patient with an IgG1 paraprotein in the serum. These cells were morphologically and immunologically mature plasma cells. They lacked the B-cell markers surface Ig, CD19 and HLA-Dr, and were positive for HAN-PC1 and CD38. The cells were weakly positive for anti-J-chain and did not bind secretory component, indicating that the IgM was monomeric or aberrant. These findings show that class switch by deletion of DNA regions in mature plasma cells is not obligatory for the secretion of immunoglobulin classes other than IgM.