We briefly report antiacetylcholine receptor antibody production by cultured bone marrow cells (4.9 +/- 0.9 fmol/10(6) cells/wk, mean +/- SD) in a 65-year-old man with myasthenia gravis without thymoma. Lymphocytes from peripheral blood, thymus, and lymph nodes produced less antibody (1.8 +/- 0.1, 0.9 +/- 0.3, and 1.0 +/- 0.4, respectively). The thymus was involuted, and the effect of thymectomy was doubtful 6 months postoperatively.