In vitro functional properties of peripheral blood mononuclear cells were evaluated in 29 patients with myasthenia gravis and in 11 healthy controls. Spontaneous cell proliferation was higher in patients than in controls. The production of interleukin-2 and interferon-gamma and the proliferative response to different mitogens were reduced in the patients. A positive correlation was found between the production of interleukin-2 and interferon-gamma. These defects in T cell function were the most pronounced in nonthymectomized patients. Patients with severe disease had a higher percentage of cells bearing the interleukin-2 receptor and a higher spontaneous production of tumor necrosis factor alpha in cell culture than in patients with mild disease. There was no difference between patients and controls in the level of soluble interleukin-2 receptor in cell culture supernatants or in sera. The results indicate a partially suppressed T cell function in myasthenia gravis. This defect was less pronounced in patients studied after thymectomy.