Peripheral blood lymphocytes from 14 patients with systemic lupus erythematosus, 5 patients with rheumatoid arthritis, and 10 normal subjects were cultured for 7 days with or without anti‐IgM or anti‐IgD antibodies, and IgG‐ and IgM‐secreting cells were assayed by reverse hemolytic plaque assay. Surface immunoglobulin (Ig) isotypes on peripheral blood B cells were also examined by a direct anti‐Ig resetting reaction. In normal subjects and rheumatoid arthritis patients, the spontaneous development of IgG‐ and IgM‐secreting cells was markedly suppressed by anti‐IgM or anti‐IgD antibodies. Over 50% of peripheral blood B cells were IgD‐ and/or IgM‐bearing cells in normal subjects and in most patients with rheumatoid arthritis. In lupus patients, however, the suppression of IgG and IgM production by anti‐IgM or anti‐IgD antibodies was remarkably reduced, especially in the active stage. Furthermore, the percentage of IgD‐bearing cells in peripheral blood B cells was remarkably reduced, especially in patients with active disease. There was a good correlation between reduced susceptibility of B cells to anti‐IgM antibody‐mediated suppression and reduced percentage of IgD‐bearing cells in lupus patients.