The in vitro T-cell dependent B-cell activity was analysed in 34 patients with myasthenia gravis and in 16 controls by culturing 1 X 10(6) peripheral blood lymphocytes for 8 days with or without pokeweed mitogen (PWM) and measuring the amounts of IgG and IgM released into the culture supernatant. Increased production of IgG in the unstimulated cultures was found in 20 patients, 8 of whom also produced increased amounts of IgM. Upon PWM stimulation the patients produced normal amounts of IgG. The IgM production, however, was decreased in the patient population compared to the controls. Only one of the controls produced more IgG than IgM, either unstimulated or during PWM stimulation. In contrast 10 patients produced more IgG than IgM in the unstimulated cultures and 15 patients did so during PWM stimulation. The 'spontaneous' production of immunoglobulins was not measurable before 4 days of culture and was not the result of in vivo activated B-cells. We suggest that the increased 'spontaneous' immunoglobulin production and the decreased production of IgM during PWM stimulation may be a result of abnormal T-cell activity. The abnormalities found were not related to disease activity, the presence of multiple autoantibodies in the serum, steroid therapy or previous thymectomy. Besides these abnormalities, patients not treated with immunosuppressive drugs also showed decreased PWM induced IgG synthesis.