In vitro immunoglobulin (Ig) synthesis was studied using peripheral blood mononuclear cells (PBM) from 12 patients with myasthenia gravis (MG). In cultured PBM, levels of secreted IgG, IgM and IgA measured by ELISA, as well as the number of IgG-, IgM-, and IgA-secreting cells determined by reversed plaque assay, were higher in MG than in controls; this was not dependent upon the stimulation of PBM with pokeweed mitogen (PWM). PBM from 9 of 12 MG patients synthesized anti-acetylcholine receptor (AChR) antibodies in vitro in the presence of PWM, while PBM from 7 controls did not. Patients whose PBM secreted lower levels of each Ig class also secreted fewer or no anti-AChR antibodies. Levels of anti-AChR antibodies secreted in vitro showed a high degree of correlation with titers of sera from patients (r = 0.86). The results suggest that the process of anti-AChR antibody synthesis by PBM in vitro is related to that of Ig synthesis, and that PBM plays, at least in part, an important role in the production of anti-AChR antibodies in MG.